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From Biosynthesis to total synthesis
From Biosynthesis to total synthesis
strategies and tactics for natural Products
Edited by
alexandros l ZograFosAristotle University of Thessaloniki Greece
Copyright copy 2016 by John Wiley amp Sons Inc All rights reserved
Published by John Wiley amp Sons Inc Hoboken New JerseyPublished simultaneously in Canada
No part of this publication may be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechanical photocopying recording scanning or otherwise except as permitted under Section 107 or 108 of the 1976 United States Copyright Act without either the prior written permission of the Publisher or authorization through payment of the appropriate per‐copy fee to the Copyright Clearance Center Inc 222 Rosewood Drive Danvers MA 01923 (978) 750‐8400 fax (978) 750‐4470 or on the web at wwwcopyrightcom Requests to the Publisher for permission should be addressed to the Permissions Department John Wiley amp Sons Inc 111 River Street Hoboken NJ 07030 (201) 748‐6011 fax (201) 748‐6008 or online at httpwwwwileycomgopermissions
Limit of LiabilityDisclaimer of Warranty While the publisher and author have used their best efforts in preparing this book they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose No warranty may be created or extended by sales representatives or written sales materials The advice and strategies contained herein may not be suitable for your situation You should consult with a professional where appropriate Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages including but not limited to special incidental consequential or other damages
For general information on our other products and services or for technical support please contact our Customer Care Department within the United States at (800) 762‐2974 outside the United States at (317) 572‐3993 or fax (317) 572‐4002
Wiley also publishes its books in a variety of electronic formats Some content that appears in print may not be available in electronic formats For more information about Wiley products visit our web site at wwwwileycom
Library of Congress Cataloging‐in‐Publication Data
Names Zografos Alexandros L editorTitle From biosynthesis to total synthesis strategies and tactics for natural products edited by Alexandros L ZografosDescription Hoboken New Jersey John Wiley amp Sons Inc [2016] | Includes bibliographical references and indexIdentifiers LCCN 2015037375 (print) | LCCN 2015047240 (ebook) | ISBN 9781118751732 (cloth) | ISBN 9781118753569 (Adobe PDF) | ISBN 9781118753637 (ePub)Subjects LCSH Organic compoundsndashSynthesis | BiosynthesisClassification LCC QD262 F76 2016 (print) | LCC QD262 (ebook) | DDC 57245ndashdc23LC record available at httplccnlocgov2015037375
Set in 1012pt Times by SPi Global Pondicherry India
Printed in the United States of America
10 9 8 7 6 5 4 3 2 1
Dedicated to my mother father and wife
LIST OF CONTRIBUTORS xiii
PREFACE xv
1 From Biosyntheses to Total Syntheses An Introduction 1Bastien Nay and Xu‐Wen Li
11 From Primary to Secondary Metabolism the Key Building Blocks 1111 Definitions 1112 Energy Supply and Carbon Storing at the Early Stage
of Metabolisms 1113 Glucose as a Starting Material toward Key Building Blocks
of the Secondary Metabolism 1114 Reactions Involved in the Construction of Secondary Metabolites 3115 Secondary Metabolisms 4
12 From Biosynthesis to total Synthesis Strategies toward the Natural Product Chemical Space 10121 the Chemical Space of Natural Products 10122 the Biosynthetic Pathways as an Inspiration
for Synthetic Challenges 11123 the Science of total Synthesis 14124 Conclusion a Journey in the Future of total Synthesis 16
References 16
SECTION I ACETATE BIOSYNTHETIC PATHWAY 19
2 Polyketides 21Franccediloise Schaefers Tobias A M Gulder Cyril Bressy Michael Smietana Erica Benedetti Stellios Arseniyadis Markus Kalesse and Martin Cordes
21 Polyketide Biosynthesis 21211 Introduction 21212 assembly of acetateMalonate‐Derived Metabolites 23213 Classification of Polyketide Biosynthetic Machineries 23214 Conclusion 39
References 40
CONTENTS
viii CoNtENtS
22 Synthesis of Polyketides 44221 asymmetric alkylation Reactions 44222 applications of asymmetric alkylation Reactions in total Synthesis
of Polyketides and Macrolides 60References 8323 Synthesis of Polyketides‐Focus on Macrolides 87
231 Introduction 87232 Stereoselective Synthesis of 13‐Diols asymmetric aldol Reactions 88233 Stereoselective Synthesis of 13‐Diols asymmetric Reductions 106234 application of Stereoselective Synthesis of 13‐Diols in
the total Synthesis of Macrolides 117235 Conclusion 126
References 126
3 Fatty Acids and Their Derivatives 130Anders Vik and Trond Vidar Hansen
31 Introduction 13032 Biosynthesis 130
321 Fatty acids and Lipids 130322 Polyunsaturated Fatty acids 134323 Mediated oxidations of ω‐3 and ω‐6 Polyunsaturated
Fatty acids 13533 Synthesis of ω‐3 and ω‐6 all‐Z Polyunsaturated Fatty acids 140
331 Synthesis of Polyunsaturated Fatty acids by the Wittig Reaction or by the Polyyne Semihydrogenation 140
332 Synthesis of Polyunsaturated Fatty acids via Cross Coupling Reactions 143
34 applications in total Synthesis of Polyunsaturated Fatty acids 145341 Palladium‐Catalyzed Cross Coupling Reactions 145342 Biomimetic transformations of Polyunsaturated Fatty acids 149343 Landmark total Syntheses 153344 Synthesis of Leukotriene B
5 158
35 Conclusion 160acknowledgments 160References 160
4 Polyethers 162Youwei Xie and Paul E Floreancig
41 Introduction 16242 Biosynthesis 162
421 Ionophore antibiotics 162422 Marine Ladder toxins 165423 annonaceous acetogenins and terpene Polyethers 165
43 Epoxide Reactivity and Stereoselective Synthesis 166431 Regiocontrol in Epoxide‐opening Reactions 166432 Stereoselective Epoxide Synthesis 172
44 applications to total Synthesis 176441 acid‐Mediated transformations 176442 Cascades via Epoxonium Ion Formation 179443 Cyclizations under Basic Conditions 181444 Cyclization in Water 182
45 Conclusions 183References 184
CoNtENtS ix
SECTION II MEVALONATE BIOSYNTHETIC PATHWAY 187
5 From Acetate to Mevalonate and Deoxyxylulose Phosphate Biosynthetic Pathways An Introduction to Terpenoids 189Alexandros L Zografos and Elissavet E Anagnostaki
51 Introduction 18952 Mevalonic acid Pathway 19153 Mevalonate‐Independent Pathway 19254 Conclusion 194References 194
6 Monoterpenes and Iridoids 196Mario Waser and Uwe Rinner
61 Introduction 19662 Biosynthesis 196
621 acyclic Monoterpenes 197622 Cyclic Monoterpenes 197623 Iridoids 200624 Irregular Monoterpenes 202
63 asymmetric organocatalysis 203631 Introduction and Historical Background 204632 Enamine Iminium and Singly occupied Molecular
orbital activation 207633 Chiral (Broslashnsted) acids and H‐Bonding Donors 213634 Chiral BroslashnstedLewis Bases and Nucleophilic Catalysis 218635 asymmetric Phase‐transfer Catalysis 220
64 organocatalysis in the total Synthesis of Iridoids and Monoterpenoid Indole alkaloids 225641 (+)‐Geniposide and 7‐Deoxyloganin 226642 (ndash)‐Brasoside and (ndash)‐Littoralisone 227643 (+)‐Mitsugashiwalactone 229644 alstoscholarine 229645 (+)‐aspidospermidine and (+)‐Vincadifformine 230646 (+)‐Yohimbine 230
65 Conclusion 231References 231
7 Sesquiterpenes 236Alexandros L Zografos and Elissavet E Anagnostaki
71 Biosynthesis 23672 Cycloisomerization Reactions in organic Synthesis 244
721 Enyne Cycloisomerization 245722 Diene Cycloisomerization 257
73 application of Cycloisomerizations in the total Synthesis of Sesquiterpenoids 266731 Picrotoxane Sesquiterpenes 266732 aromadendrane Sesquiterpenes Epiglobulol 267733 CubebolndashCubebenes Sesquiterpenes 267734 Ventricos‐7(13)‐ene 270735 Englerins 271736 Echinopines 271737 Cyperolone 273
x CoNtENtS
738 Diverse Sesquiterpenoids 27674 Conclusion 276References 276
8 Diterpenes 279Louis Barriault
81 Introduction 27982 Biosynthesis of Diterpenes Based on Cationic Cyclizations
12‐Shifts and transannular Processes 27983 Pericyclic Reactions and their application in the Synthesis
of Selected Diterpenoids 284831 Dielsndashalder Reaction and Its application in the total
Synthesis of Diterpenes 284832 Cascade Pericyclic Reactions and their application in the total
Synthesis of Diterpenes 29184 Conclusion 293
References 294
9 Higher Terpenes and Steroids 296Kazuaki Ishihara
91 Introduction 29692 Biosynthesis 29693 Cascade Polyene Cyclizations 303
931 Diastereoselective Polyene Cyclizations 303932 ldquoChiral proton (H+)rdquo‐Induced Polyene Cyclizations 304933 ldquoChiral Metal Ionrdquo‐Induced Polyene Cyclizations 308934 ldquoChiral Halonium Ion (X+)rdquo‐Induced Polyene Cyclizations 313935 ldquoChiral Carbocationrdquo‐Induced Polyene Cyclizations 319936 Stereoselective Cyclizations of Homo(polyprenyl)arene
analogs 31994 Biomimetic total Synthesis of terpenes and Steroids through
Polyene Cyclization 31995 Conclusion 328
References 328
SECTION III SHIKIMIC ACID BIOSYNTHETIC PATHWAY 331
10 Lignans Lignins and Resveratrols 333Yu Peng
101 Biosynthesis 3331011 Primary Metabolism of Shikimic acid and aromatic
amino acids 3331012 Lignans and Lignin 335
102 auxiliary‐assisted C(sp3)ndashH arylation Reactions in organic Synthesis 336
103 FriedelndashCrafts Reactions in organic Synthesis 344104 total Synthesis of Lignans by C(sp3)H arylation Reactions 353105 total Synthesis of Lignans and Polymeric Resveratrol by
FriedelndashCrafts Reactions 357106 Conclusion 375References 375
CoNtENtS xi
SECTION IV MIXED BIOSYNTHETIC PATHWAYSndash THE STORY OF ALKALOIDS 381
11 Ornithine and Lysine Alkaloids 383Sebastian Brauch Wouter S Veldmate and Floris P J T Rutjes
111 Biosynthesis of l‐ornithine and l‐Lysine alkaloids 3831111 Biosynthetic Formation of alkaloids
Derived from l‐ornithine 3831112 Biosynthetic Formation of alkaloids
Derived from l‐Lysine 388112 the asymmetric Mannich Reaction in organic Synthesis 392
1121 Chiral amines as Catalysts in asymmetric Mannich Reactions 3941122 Chiral Broslashnsted Bases as Catalysts in asymmetric
Mannich Reactions 3981123 Chiral Broslashnsted acids as Catalysts in asymmetric
Mannich Reactions 4041124 organometallic Catalysts in asymmetric Mannich Reactions 4081125 Biocatalytic asymmetric Mannich Reactions 413
113 Mannich and Related Reactions in the total Synthesis of l‐Lysine‐ and l‐ornithine‐Derived alkaloids 414
114 Conclusion 426References 427
12 Tyrosine Alkaloids 431Uwe Rinner and Mario Waser
121 Introduction 431122 Biosynthesis of tyrosine‐Derived alkaloids 431
1221 Phenylethylamines 4311222 Simple tetrahydroisoquinoline alkaloids 4331223 Modified Benzyltetrahydroisoquinoline alkaloids 4331224 Phenethylisoquinoline alkaloids 4361225 amaryllidaceae alkaloids 4381226 Biosynthetic overview of tyrosine‐Derived alkaloids 442
123 arylndasharyl Coupling Reactions 4421231 Copper‐Mediated arylndasharyl Bond Forming Reactions 4431232 Nickel‐Mediated arylndasharyl Bond Forming Reactions 4461233 Palladium‐Mediated arylndasharyl Bond Forming Reactions 4471234 transition Metal‐Catalyzed Couplings of Nonactivated
aryl Compounds 450124 Synthesis of tyrosine‐Derived alkaloids 456
1241 Synthesis of Modified Benzyltetrahydroisoquinoline alkaloids 4561242 Synthesis of Phenethylisoquinoline alkaloids 4601243 Synthesis of amaryllidaceae alkaloids 462
125 Conclusion 468References 469
13 Histidine and Histidine‐Like Alkaloids 473Ian S Young
131 Introduction 473132 Biosynthesis 473133 atom Economy and Protecting‐Group‐Free Chemistry 480
xii CoNtENtS
134 Challenging the Boundaries of Synthesis PIas 488135 Conclusion 497References 499
14 Anthranilic AcidndashTryptophan Alkaloids 502Zhen‐Yu Tang
141 Biosynthesis 502142 Divergent SynthesisndashCollective total Synthesis 508143 Collective total Synthesis of tryptophan‐Derived alkaloids 510
1431 Monoterpene Indole alkaloids 5101432 Bisindole alkaloids 512
References 517
15 Future Directions of Modern Organic Synthesis 519Jakob Pletz and Rolf Breinbauer
151 Introduction 519152 Enzymes in organic Synthesis Merging total
Synthesis with Biosynthesis 520153 Engineered Biosynthesis 526154 Diversity‐oriented Synthesis Biology‐oriented Synthesis
and Diverted total Synthesis 5331541 Diversity‐oriented Synthesis 5351542 Biology‐oriented Synthesis 5361543 Diverted total Synthesis 539
155 Conclusion 541References 545
INDEX 548
Elissavet E Anagnostaki Department of Chemistry Laboratory of Organic Chemistry Aristotle University of Thessaloniki Thessaloniki Greece and Research and Development Department Pharmathen SA Thessaloniki Greece
Stellios Arseniyadis School of Biological and Chemical Sciences Queen Mary University of London London United Kingdom
Louis Barriault Department of Chemistry University of Ottawa Ottawa Ontario Canada
Erica Benedetti Laboratoire de Chimie et Biochimie et Pharmacologiques et Toxicologiques CNRS-Universiteacute Paris Descartes Faculteacute des Sciences Fondamentales et Biomeacutedicales Paris France
Sebastian Brauch Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
Rolf Breinbauer Institute of Organic Chemistry Technische Universitaumlt Graz Graz Austria
Cyril Bressy Aix Marseille Universiteacute Centrale Marseille CNRS Marseille France
Martin Cordes Institute for Organic Chemistry and Center of Biomolecular Drug Research (BMWZ) Leibniz Universitaumlt Hannover Hannover Germany and Helmholtz Center for Infection Research (HZI) Hannover Germany
Paul E Floreancig Department of Chemistry Chevron Science Center University of Pittsburgh Pittsburgh PA USA
Tobias A M Gulder Department of Chemistry and Center for Integrated Protein Science Munich (CIPSM) Biosystems Chemistry Technische Universitaumlt Muumlnchen Munich Germany
Trond Vidar Hansen School of Pharmacy University of Oslo Oslo Norway
Kazuaki Ishihara Department of Biotechnology Graduate School of Engineering Nagoya University Nagoya Japan
Markus Kalesse Institute for Organic Chemistry and Center of Biomolecular Drug Research (BMWZ) Leibniz Universitaumlt Hannover Hannover Germany and Helmholtz Center for Infection Research (HZI) Hannover Germany
Xu‐Wen Li Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
Bastien Nay Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France
Yu Peng State Key Laboratory of Applied Organic Chemistry Lanzhou University Lanzhou China
Jakob Pletz Institute of Organic Chemistry Technische Universitaumlt Graz Graz Austria
Uwe Rinner Institute of Organic Chemistry Johannes Kepler University Linz Linz Austria and Department of Chemistry College of Science Sultan Qaboos University Muscat Oman
Floris P J T Rutjes Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
LIST oF CoNTRIBUToRS
xiv LIST OF CONTRIBUTORS
Franccediloise Schaefers Department of Chemistry and Center for Integrated Protein Science Munich (CIPSM) Biosystems Chemistry Technische Universitaumlt Muumlnchen Munich Germany
Michael Smietana Institut des Biomoleacutecules Max Mousseron CNRS Universiteacute de Montpellier ENSCM France
Zhen‐Yu Tang Department of Pharmaceutical Engineering College of Chemistry and Chemical Engineering Central South University Changsha China
Wouter S Veldmate Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
Anders Vik School of Pharmacy University of Oslo Oslo Norway
Mario Waser Institute of Organic Chemistry Johannes Kepler University Linz Linz Austria
Youwei Xie Max‐Planck‐Institut fuumlr Kohlenforschung Muumllheim Germany
Ian S Young Bristol‐Myers Squibb Company Chemical Development New Brunswick NJ USA
Alexandros L Zografos Department of Chemistry Laboratory of Organic Chemistry Aristotle University of Thessaloniki Thessaloniki Greece
There is pleasure in the pathless woodsthere is rapture in the lonely shore
there is society where none intrudesby the deep sea and music in its roar
I love not Man the less but Nature moreLord Byron
Preface
The first time I came across with the idea of editing a book that merges selected chapters of biosynthesis and total synthesis was when I was teaching postgraduate courses of natural product synthesis at Aristotle University of Thessaloniki This period I realized that the best way to teach youngsters synthesis was to start from the very origin of inspiration nature and its tools biosynthesis
Over the last decades biosynthesis is filling our gaps of understanding the complex mechanisms of nature and pro-vides useful sources of inspiration not only in the way natural products can be synthesized but also by directing synthetic chemists in developing atom‐economical efficient synthetic methods Several are the examples that mimic biosynthetic guidelines from modern iterative alkylations and aldol reactions to CH oxidations that compile nowadays the modern toolbox of organic synthesis
The handed book is constructed in the logic of presenting the parallel development of biosynthesis and organic meth-odology and how these can be applied in efficient syntheses of natural products The book is divided into four sections each representing the four major biosynthetic pathways of natural products namely acetate mevalonate shikimate biosynthetic pathways and the mixed biosynthetic pathways
of alkaloids These sections are divided into chapters that represent selected classes of natural products for example lipids sesquiterpenoids lignans etc Each of these chapters is further divided into three distinct subchapters (a) biosyn-thesis (b) methodological section and (c) application of the described methodology in the total synthesis of the described family of natural products By this way the readers can be focused in the direct comparison between biosyn-theses and the developed methodologies to construct the crucial for each class of natural product carbon bonds Although the book as it develops is focused on presenting the power of biosynthesis and how this power can be applied in providing inspiration for the efficient synthesis of natural products it was not the authors will to present only biomi-metic total syntheses but rather to exploit the modern synthetic methodologies and recognize their disabilities for further improvement
Of course this book will not have been realized without the excellent work of renowned scientists worldwide working either in the field of biosynthesis or total synthesis who collected the existing knowledge on biosynthesis analyzed the existing modern methodologies and presented a bouquet of selected total syntheses Throughout our
xvi PrEfACE
endeavor to complete this book I learned many things from their expertise but I also realized that only with tight collab-orations you can build long‐lasting friendships I would like to thank them all once again for their trust and effort to complete this book We all hope that the current work will contribute to a better understanding of the current status of
organic chemistry and to the discovery of novel strategies and tactics for the synthesis of natural products
Alexandros L ZografosSeptember 2015
Thessaloniki Greece
From Biosynthesis to Total Synthesis Strategies and Tactics for Natural Products First Edition Edited by Alexandros L Zografoscopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
11 FROM PRIMARY TO SECONDARY METABOLISM THE KEY BUILDING BLOCKS
111 Definitions
The primary and secondary metabolisms are traditionally distinguished by their distribution and utility in the living organism network Primary metabolites include carbohyshydrates lipids nucleic acids and proteins (or their amino acid constituents) and are shared by all living organisms on Earth They are transformed by common pathways which are studied by biochemistry (Fig 11) Secondary metabo-lites are structurally diverse compounds usually produced by a limited number of organisms which synthesize them for a special purpose like defense or signaling through specific biosynthetic pathways They are studied by natural product chemistry This distinction is not always so obvious and some compounds can be studied in the context of both primary and secondary metabolisms This is especially true nowadays with the use of genetic and biomolecular tools which tend to make natural product sciences more and more integrative However an important point to remember is that the primary metabolism furnishes key building blocks to the secondary metabolism It would be difficult to describe in detail the full biosynthetic pathshyways in this section We tried to organize the discussion as a vade mecum synthetically gathering information from extremely useful sources which will be cited at the end of this chapter
112 Energy Supply and Carbon Storing at the Early Stage of Metabolisms
The sunlight is essential to life except in some part of the deep oceans It provides energy for plant photosynthesis that splits molecules of water into protons and electrons and releases O
2 (Scheme 11) A proton gradient inside the plant
chloroplasts then drags a transmembrane ATP synthase comshyplex that produces adenosine triphosphate (ATP) while elecshytrons released from water are transferred to the coenzyme reducer nicotinamide adenine dinucleotide phosphate hydride (NADPH) A major function of chloroplasts is to fix CO
2 as a combination to ribulose‐15‐bisphosphate (RuBP)
performed by RuBP carboxylase (rubisco) forming an instable ldquoC
6rdquo β‐ketoacid This is cleaved into two molecules
of 3‐phosphoglycerate (3‐PGA) which is then reduced into 3‐phosphoglyceraldehyde (3‐PGAL a ldquoC
3rdquo triose phosshy
phate) during the Calvin cycle This is one of the major metabolites in the biosynthesis of carbohydrates like glucose and a biochemical mean for storing and retaining carbon atoms in the living cells
113 Glucose as a Starting Material Toward Key Building Blocks of the Secondary Metabolism
Glucose‐6‐phosphate arises from the phosphorylation of glucose It is the starting material of glycolysis an important process of the primary metabolism which consists in eight enzymatic reactions leading to pyruvic acid (PA)
FROM BIOSYNTHESES TO TOTAL SYNTHESES AN INTRODUCTION
Bastien Nay1 and Xu‐Wen Li2
1
1 Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France2 Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
2 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
(Scheme 12) Important intermediates for the secondary metabolism are produced during glycolysis Glucose glucose‐6‐phosphate and fructose‐6‐phosphate can be converted to other hexoses and pentoses that can be oligoshymerized and enter in the composition of heterosides Additionally fructose‐6‐phosphate connects the pentose
phosphate pathway leading to erythrose‐4‐phosphate toward shikimic acid which is a key metabolite in the biosynthesis of aromatic amino acids (phenylalanine tyrosine or C
6C
3
units) and C6C
1 phenolic compounds The next important
intermediate in glycolysis is 3‐PGAL which can be redishyrected toward methylerythritol‐4‐phosphate (mEP) in the
Primary metabolism Secondary metabolism
The field ofbiochemistry
The field ofnatural product
chemistry
Essential toliving organisms
Essential to theproducer organisms
under particularconditions
Nucleic acids (DNARNA) carbohydrates
lipids amino acidspeptides and proteins
Alkaloids terpenespolyketides
polyphenols andtheir heterosidic form
Biological effects(defense signaling)
Biosyntheticpathways
Main compoundclasses
FIGURE 11 Primary versus secondary metabolisms
PS-I
PS-IIendash
hν H2O H+ and O2
ATP synthase
NADP reductase
H+ (gradient)
H+ADP + Pi ATP
NADPHH+NADP+
H+
Thylakoidcompartment
Chloroplaststroma
Thylakoidmembrane
(a) Light dependent process
(b) Light independent process
CO2
RuBP
Rubisco
Unstable β-ketoacid3-PGA 13-diPGA
ATP
ADP3-PGAL
NADPHH+
NADP+
(Calvin cycle)
trioses tetroses pentoses hexoses (eg glucose) heptoses
Chloroplaststroma
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
CH2OPO32ndash CH2OPO3
2ndash CH2OPO32ndash
CH2OPO32ndash
OOH
HO
HOO
HO
HO2CCO2H
HO
CO2PO32ndash
HO
CHOHO
endash
Cytosol
SCHEME 11 The photosynthetic machinery (PS‐I and PS‐II photosystems I and II)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 3
chloroplast mEP is a starting block in the biosynthesis of terpenes through C
5 isoprene units (isopentenyl diphosphate
(IPP) and dimethylallyl diphosphate (DmAPP)) especially those in C
10 C
20 and C
40 terpenes 3‐PGA is a precursor of
serine and other amino acids while phosphoenolpyruvate (PEP) the precursor of PA is also an intermediate toward the previously mentioned shikimic acid Lastly PA is not only a precursor of the fundamental ldquoC
2rdquo acetyl coenzyme A
(AcCoA) unit but also an intermediate toward aliphatic amino acids and mEP
AcCoA is the building block of fatty acids polyketides and mevalonic acid (mVA) a cytosolic precursor of the C
5 isoprene units for the biosynthesis of terpenes in the
C15
and C30
series (mind it is different from the mEP pathway in product and in cell location) Finally AcCoA enters the citric acid or Krebs cycle which leads to several precursors of amino acids These are oxaloacetic acid precursor of aspartic acid through transamination (thus toward lysine as a nitrogenated C
5N linear unit and methishy
onine as a methyl supplier) and 2‐oxoglutaric acid preshycursor of glutamic acid (and subsequent derivatives such as ornithine as a nitrogenated C
4N linear unit) All these
amino acids are key precursors in the biosynthesis of many alkaloids
114 Reactions Involved in the Construction of Secondary Metabolites
most reactions occurring in the living cells are performed by specialized enzymes which have been classified in an intershynational nomenclature defined by an enzyme commission (EC) number There are six classes of enzymes depending on the biochemical reaction they catalyze EC‐1 oxidoreducshytases (catalyzing oxidoreduction reactions) EC‐2 transfershyases (catalyzing the transfer of functional groups) EC‐3 hydrolases (catalyzing hydrolysis) EC‐4 lyases (breaking bonds through another process than hydrolysis or oxidation leading to a new double bond or a new cycle) EC‐5 isomershyases (catalyzing the isomerization of a molecule) and EC‐6 ligases (forming a covalent bond between two molecules) many subclasses of these enzymes have been described depending on the type of atoms and functional groups involved in the reaction and if any on the cofactor used in this reaction For example several cofactors can be used by dehydrogenases like NAD(P)NAD(P)H FADFADH
2 or
FmNFmNH2 For a description of this classification the
reader can refer to specialized Internet websites like ExplorEnz [1] What is important to realize is that most enzymes are substrate specific and have been selected during
CHOHO
CO2H
O
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
HOOH
HO
HOH2CO
OHC
CH2OPO32ndash
OHHO
CO2H
HO
OH
OH
TYR
PHE
CO2H
NH2
TRP
CH2OPO32ndash
HO
HOH2COH
MEP
C10 C20 and C40 terpenes
CO2HHO
SER
GLY CYS
CO2HOPO3
2ndash
CH2OH
OHHO2C
MVA
C15 and C30 terpenes
Polyketides
Krebscycle
(citric acid)
VAL ALA ILE LEU
CO2H
O
HO2C
CO2HO
CO2H
ASP
LYS
GLU
Glucose-6-phosphate
Fructose-6-phosphate
Erythrose-4-phosphate
3-PGAL
OP2O63ndash OP2O6
3ndash
IPP DMAPP
3-PGA PEP PA
O SCoAAcCoA
3x
Cytosol
Cytosol
Chloroplasts
Chloroplasts
Glycolysis
C2
C5
Shikimic acid Oxaloaceticacid
2-Oxoglutaricacid
MET
THR
PRO
ARGORN
Alkaloids Alkaloids Alkaloids Alkaloids
Aminoacids
Peptidesproteins
Phenolics
C1
C5N C4N
ldquoCH3rdquo
(Indole)C2NC6C3C6C2N C6C1
Pentosephosphatepathway
SCHEME 12 The building block chart involving glycolysis and the Krebs cycle
4 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
evolution to perform specific transformations making natural products with often and yet unknown functions
Secondary metabolites arise from specific biosynthetic pathways which use the previously defined building blocks The bunch of organic reactions involved in these biosynshytheses allows the construction of natural product frameshyworks which are finally diversified through ldquodecorationrdquo steps (Scheme 13) It is not the purpose of this introductive chapter to describe in detail all biosynthetic pathways and the reader can refer to excellent books and articles which have been published elsewhere [2 3]
The reactions involved in the construction of natural product skeletons will be described later for representative classes of compounds The identification of the building block footprint in the natural product skeleton will be emphasized as much as possible sometimes referring to biogenetic speculations [4] After the framework construction the decoration steps will involve as diverse reactions as aliphatic CH oxidations (eg involving a cytochrome P
450 oxygenase) occasionally triggering a
rearrangement heteroatom alkylations (eg methylation by S‐adenosylmethionine) or allylation (by DmAPP) esterifications heteroatom or C‐glycosylations (leading to heterosides) radical couplings (especially for phenols) alcohol oxidations or ketone reductions amineketone transaminations alkene dihydroxylations or epoxidations oxidative halogenations BaeyerndashVilliger oxidations and further oxygenation steps At the end of the biosynthesis such transformations may totally hide the primary building block origin of natural products
115 Secondary Metabolisms
1151 Polyketides Polyketides (or polyacetates) are issued from the oligomerization of C
2 acetate units performed
by polyketide synthases (PKS) and leading to (C2)
n linear
intermediates [5 6] If the (C2)
n intermediates arise from
successive Claisen reactions performed by ketosynthase
domains (KS in nonreducing PKS) a highly reactive poly‐ β‐ketoacyl intermediate H(CH
2CO)
nOH is formed
leading to phenolic and aromatic products through further intramolecular Claisen condensations Furthermore highly reducing PKSs are made of specialized enzymatic subunits working in line or iteratively to functionalize each C
2 linker
bond as CH(OH)CH2 (by ketoreductases (KR)) then as
HCCH (by dehydratases (DH)) and as CH2CH
2 (by enoyl
reductases (ER)) leading to a high degree of functionalization of the final product (Fig 12) By these iterative sequences highly reduced polyketides which can be either linear macrocyclized or polycyclized depending on the reactivity of the biosynthetic intermediates can be formed [7] With the same logic fatty acids are also biosynthesized by fatty acid synthases
moreover the PKS enzyme can be hybridized with nonshyribosomal peptide synthetase (NRPS) domains (see also ldquoNRPS metabolites and peptidesrdquo in the ldquoAlkaloidsrdquo secshytion) leading to the acylation of an amino acid by the (C
2)
n
acyl intermediate As previously the functionalization of the acyl chain depends on the PKS enzyme and the PKSNRPS products are also extremely diversified (eg hirsutellone B Fig 12) [8]
1152 Terpenes Terpenes are derived from the oligoshymerization of the C
5 isoprene units DmAPP and IPP Both
precursors are prompt to generate either an allylic cation (the diphosphate is a good leaving group) or a tertiary carbocation respectively which makes the IPP easy to react with DmAPP (Scheme 14) This reaction happens in the active site of a terpene synthase which activates the deparshyture of the diphosphate group from DmAPP thanks to Lewis acid activation (a metal like mg2+ mn2+ or Co2+ is present in the enzyme active site [9]) This leads to geranyl (C
10
monoterpene precursor) or farnesyl (C15
sesquiterpene precursor) diphosphate depending on the location of the enzyme (chloroplast for the mEP pathway or cytosol for the mVA pathway) Geranylgeranyl (C
20 diterpene) and
Constructionreactions
Decoration reaction(functionalization)Building
blocksNatural product
frameworksNatural products
HH
OH OAcH
HO O OH
OHO
OBz
P450
P450
P450
P450
P450 P450 Oxidativeauxiliaryenzymes
Taxadiene
OP2O63ndash
OP2O63ndash
3times
(a)
(b)
10-Deacetylbaccatin III
DMAPP
IPPand electrophilic
cyclizations
SCHEME 13 (a) From building blocks to natural products and (b) the example of 10‐deacetylbaccatin III
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 5
farnesylfarnesyl (C30
triterpene precursor) diphosphates can also be obtained by further additions of IPP leading to longer linear intermediates
The cyclization of linear precursors is achieved by speshycialized cyclases which generate a poorly functionalized natural product framework [10 11] Auxiliary enzymes such as oxygenases then increase the complexity and the diversity
of compounds by further functionalization (Scheme 13b) [12] A high degree of oxidation can be observed in comshypounds like thapsigargin paclitaxel or bilobalide (Fig 13) The biosynthesis of this last compound for example involves a high oxygenation pattern two Wagnerndashmeerwein rearrangements and several oxidative cleavages leading to the loss of five carbons The resulting natural products can
KS
S-ACP
O O
KR
S-ACP
OH O
S-MAT
O
YesNo
R
R
R
YesNoDH
S-ACP
OR
YesNoER
S-ACP
OR
YesNoTE
OH
OR
New cycle
New cycleor
release
New cycleor
release
New cycleor
release
Release
R in
crem
ente
d by
two
carb
ons
HO
O
S-ACP
O
Claisen condensation (ndashCO2)
reduction (NADPH)
dehydration (ndashH2O)
+
reduction (NADPH)
hydrolysis (H2O)
O O NH
OH
OH
H H
H H
Hirsutellone B(mixed PKSNRPS
product)
OO
O
OHOH
HO OH
Norsolorinic acid
O
O
O
O CHO
OH
HH
H
H
HH
OHHemibrevetoxin B
OOHO
OHO
HOHO OH
Erythronolide A
OH
OH
HO
Panaxytriol
From ahexanoylstartingblock
H
H
Fromtyrosine
H
1C lost fromdecarboxylation
FIGURE 12 Chemical logic of polyketide construction leading to variable functionalization of the elongated acyl chain and examples of resulting chemical diversity ACP acyl carrier protein DH dehydratase ER enoyl reductase KR ketoreductase KS ketosynthase mAT malonyl acyl transferase TE thioesterase
DMAPPOP2O6
3ndashOP2O6
3ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
Geranyl diphosphate (GPP)2 times IPP
H H
Geranylgeranyl diphosphate (GGPP)
Examplesverticillane
casbanetaxane
phorbol
Exampleslabdane
pimaranekauraneabietane
aphidicolanegibberellane
IPP
SCHEME 14 Early assembly of C5 units in terpene biosynthesis leading to diterpenes (C
20)
6 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
thus be extremely modified with structures whose biogenetic origin is far from being obvious at first sight and cannot be determined without further experiments such as isotopic labeling
1153 Flavonoids Resveratrols Gallic Acids and Further Polyphenolics We have previously discussed the polyketide origin of some phenolic compounds based on the (C
2)
n motif Other polyphenols like gallic acids are directly
derived by the aromatization of shikimic acid (C6C
1 building
block Scheme 12) [13] The C6C
3 building blocks are availshy
able from the conversion of phenylalanine and tyrosine into cinnamic and p‐coumaric acids respectively and then by further hydroxylation steps (Scheme 15) These can dimerize into lignans (eg podophyllotoxin) [14 15] through radical processes or converted to low molecular weight compounds like eugenol coumarins or vanillin [16] The coenzyme A thioesters of these C
6C
3 acids can be used as initiator units
by specialized ketosynthases for an elongation by two acetyl units leading to aromatic polyketides like styrylpyrones or diarylheptanoids (eg curcumin) [17] Important comshypounds from this metabolism are flavonoids (C
6C
3C
6) [18]
and stilbenoids (C6C
2C
6) (a decarboxylation occurs during
the aryl cyclization) [19] which are synthesized by chalcone synthase and stilbene synthase respectively Flavonoids (eg catechin) and stilbenes (eg resveratrol) are present in large amounts in fruits and vegetables and may exert their radical scavenging properties in vivo
1154 Alkaloids Alkaloids are nitrogen‐containing compounds The nitrogen(s) can be involved in an amine function conferring basicity to the natural product (like ldquoalkalirdquo) or in less or nonbasic functions such as an amide a nitrile an isonitrile or an ammonium salt (quaternary amines) For amines protonation often occurs at physiological pH and may condition their biological activity In many cases the nitrogen is biogenetically derived from an amino acid We will thus discuss alkaloids according to their amino acid origin
Alkaloids Derived from the Krebs Cycle (Lysine and Ornithine Derived) As shown previously (Scheme 12) the Krebs cycle is a crucial metabolic process which leads to α‐ketoacids (oxaloacetic and 2‐oxoglutaric acids) Their enzymatic transamination affords the two amino acidsmdashaspartic acid and glutamic acidmdashwhich are the direct biosynthetic precursors of amino acids lysine and ornithine respectively These in turn produce cadaverine a ldquoC
5Nrdquo unit
and putrescine a ldquoC4Nrdquo unit which are major components
for the biosynthesis of important alkaloids as will be discussed later (Scheme 16) Additionally ornithine is a precursor of arginine another important amino acid
ornithine‐derived alkaloids (incorporating the c4n
unit) Putrescine is derived from the decarboxylation of ornithine and is a precursor of linear polyamines like spermshyine After enzymatic methylation of one amine of putrescine
C10 C15
C15
C10
C10
C30
CO2H
Chrysanthemic acid
OH
Menthol
O
HO
H
HOGlc
MeO2CLoganin (iridoid)
H
H
H
HH
OO
OParthenolide
O
O
OHOHO
OAcHO
OnC3H7
O
O
O
nC7H15
Thapsigargin
O
O
OO
H
H
O
Cleavage
H
H
Artemisinin
C20 C40
O
O
OO
OO
OHOHH
Bilobalide(highly rearrangedand missing 5 C)
CleavageH
Highoxidativecleavages
HO OAcH
AcO O OH
OOOBz
O
Ph
BzNH
OHFrom PHE Paclitaxel
C25O
H
HO
OHCH
OH Ophiobolin A
H
HO
H
H
HHCholesterol
(missing 3 C WM)
H
H H H
H
H
Hopene
β-Carotene
C30 - 3
C15
C20 - 5WM
WM
FIGURE 13 Chemical diversity in the terpene series (Wm Wagnerndashmeerwein shifts lost carbons bold bonds are remnant of primary building blocks)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 7
in the presence of S‐adenosylmethionine transamination of the other affords γ‐(N‐methylamino)aldehyde [20] The resulting cyclic iminium is a key intermediate in the formation of many medicinally important alkaloids such as
the plant‐derived compounds cocaine atropine or the calysshytegines [21 22] Indeed this iminium is a mannich acceptor which can react with various nucleophiles the first of those being the carbanion of acetyl‐CoA Thus after a stepwise
CO2H
RCinnamic acid (R=H)p-Coumaric acid (R=OH)
RO
PHETYR
OH
[H] [O]
lignans
OH
O
O
O
O
OMeMeO
OMe
C mdash C andor C mdash Oradical couplings
After E Zisomerization
for example Podophyllotoxin
O ORO
Coumarins(eg scopoletin)
[O]
Prenylationcyclizationcleavage[O]
O OO
Furocoumarins(eg psoralen)
RO
nAcetyl-CoA
thencyclization
n = 2 styrylpyrones
O O
OMe
n = 3 avonoids(from chalcone synthase)
C6C3
H
H H
From AcCoA
O
n = 3 stilbenoids(from stilbene synthase)
HO
OH
OHFrom AcCoA From AcCoA(ndashCO2)
Resveratrol
HO
OH
OH
OH
OH
H
Catechin
MeO Yangonin
From AcCoA
SCHEME 15 The phenylpropanoid biosynthetic pathways
LYS
CO2
Cadaverine
O NH
H2O
NH
Piperideineiminium
Pelletierine
AcAcCoA
CO2
NH
O
N
O
Pseudopelletierine
NH
Ph
OH
Ph
O
Lobeline
NH
δ+
δndash
N
H
H
N
H
OH
Lupinine
N
H N
H
(+)-Sparteine
N
NH
O (ndash)-Cytisine
NH
CO2H
Pipecolic acid
N
OHHO
OH
HO
H
Castanos permine
ORN
CO2
NH2 NH2 NH2 NH2
NH2
NH2
NH2
PutrescineO
NHMe
N-Methylpyrrolinium
2 timesAcCoA
NMe
ARG
n = 1 spermidinen = 2 homospermidine
NMe
O
HygrineCO2
CO2
MeN
O
[O]
TropinoneCO2
HNHO
OHOH
OH
Calystegine B2
MeN
OAtropine
O
Ph
HO
NMe
NMe
NMe
O
Cuscohygrine
HN
HN
NH2
( )n
O
n = 2
N
HHO
Retronecine
HH
H
H
H
HO
H H
H
SCHEME 16 Lysine‐ and ornithine‐derived alkaloid biosynthetic pathways (mind the structural similarities)
8 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
elongation by two AcCoA units either decarboxylation can occur leading to the acetonylpyrrolidine hygrine or a secshyond mannich reaction by the intramolecular attack of the acetoacetate anion onto an oxidation‐derived pyrrolinium leading to the tropane skeleton (tropinone) The acetoacetyl‐CoA intermediate can also react intermolecularly with another pyrrolinium cation leading to cuscohygrine after decarboxylation Finally the pyrrolizidine alkaloids [23] are derived from homospermidine which when submitted to terminal oxidative deamination leads to the bicyclic skelshyeton of retronecine and further Senecio alkaloids We can mention herein that ornithine is a biosynthetic precursor of arginine bearing a guanidine function which is an intermediate toward the toxic compounds tetrodotoxin and saxitoxin (not shown)
lysine‐derived alkaloids (incorporating the c5n
unit) From lysine to piperidine alkaloids the biosynthetic steps parallel the one previously described from ornithine Indeed the oxidative deamination of cadaverine affords a δ‐amino aldehyde which cyclizes through imine formation into piperideine Protonation results in a mannich acceptor which is able to react with various nucleophiles such as β‐ketothioester anions The first product of these reactions is pelletierine which can further react through an intramolecular mannich
reaction leading to pseudopelletierine Quinolizidines [24] can also be formed first from the mannich reaction of the piperideine acceptor with the corresponding enamine nucleoshyphile and then after additional transformation steps leading for example to lupinine sparteine or cytisine
Indolizidine alkaloids [15] such as castanospermine and swainsonine are formed from pipecolic acid an amino acid derived from lysine which can be elongated by malonyl‐CoA followed by ring closure When protonated these alkaloids are oxonium mimics strongly inhibiting glycosidases
Tyrosine‐ and Phenylalanine‐Derived Alkaloids Tyrosine and phenylalanine amino acids are bearing the phenylshyethylamine moiety of many medicinally relevant alkaloids Further hydroxylations on the aromatic carbocycle or on the aliphatic part can be observed methylations can occur on phenolic oxygens and on the amine leading to catecholamines (adrenaline noradrenaline dopamine) Arylethylamines are also usual to react with endogenous aldehydes through PictetndashSpengler reactions [25] leading to important biosynthetic intermediates (Scheme 17) like
bull Reticuline from the reaction with 4‐hydroxyphenylacetshyaldehyde toward benzyltetrahydroisoquinoline alkaloids
NH2
Phenylethylamines
RONH TetrahydroisoquinolinesRO
RʹCHO
Rʹ
NH
(CH2)n
MeO
HO
Benzyltetrahydroisoquinoline(n = 1) for example reticuline
orPhenethyltetrahydroisoquinoline
(n = 2) eg autumnaline
IntermolecularCmdashO phenol
couplings
Curarealkaloids
IntramolecularC mdash C phenol
couplings
ONMe
HHO
H
HO
Morphine
NMe
MeO
HO
MeOOH
Isoboldine
O
O
OMe
NO2
CO2H
Aristolochic acid
OMe
O
NHAcMeO
MeOMeO
Colchicine
N
O
O
OMe
OMeBerberine
IntramolecularCmdash C coupling
and furtheroxidative events
Rʹ derivedfrom PHE
Pictet-Spenglerreaction
TYR
HO
HO CHO
HNHO
HO
OH
OMeO
OH
NMe
[O]
GalantamineNorbelladine
Rʹ derived fromsecologanin
NAc
HO
HO
O
CO2MeH
H
H
OHIpecosideaglycone
Ipecac alkaloids
1ClostCmdash C cleavage
and ring extension
H
ROH
1C lost
Cleavage
SCHEME 17 Tyrosine‐derived alkaloid biosynthetic pathways (double head arrows figure bond cleavages during biosynthetic processes)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 9
morphine berberine tubocurarine isoboldine or the highly modified aristolochic acid [26 27]
bull Automnaline from the reaction with 3‐(4‐hydroxyphenyl)propanal toward phenylethyltetrahydroisoquinoline alkaloids colchicine cephalotaxine or schelhammerishycine [28 29]
bull Ipecoside from the reaction of dopamine with secoloshyganin toward terpene tetrahydroisoquinoline alkaloids ipecoside or emetine
Lastly norbelladine (top of Scheme 17) is issued from the reductive amination of 34‐dihydroxybenzaldehyde (derived from phenylalanine) with tyramine (derived from tyrosine) and constitutes a biosynthetic node leading to Amaryllidaceae alkaloids such as galantamine crinine or lycorine depending on the topology of phenolic couplings In all these biosynthetic routes radical phenolic couplings are key reactions for CC and CO bond formations and rearrangements [30 31]
Tryptophan‐Derived Indole and Indole Monoterpene Alkaloids As for alkaloids derived from tyrosine and phenylalanine those derived from tryptophan are formed after decarboxylation of the amino acid (into tryptamine) and possible hydroxylation of the aromatic carbocycle (eg serotonin) and N‐methylation (eg psilocin) As previously tryptamine can also react through PictetndashSpengler reactions to form tetrahydro‐β‐carbolines which can be aromatized for example into harmine (Scheme 18) [16]
When the aldehyde partner of the PictetndashSpengler reacshytion with tryptamine is the terpene secologanin strictosidine is formed as an entry toward the vast monoterpene indole alkaloids [32 33] Hydrolysis of the glucosidic part releases the strictosidine aglycone bearing an aldehyde while iminshyium formation and further cyclization and reduction can lead to ajmalicine (from oxocyclization) or yohimbine (from carshybocyclization) These alkaloids are referred to as from the Corynanthe type with the monoterpene carbon skeleton unmodified Although it misses one carbon and has a very
RO
Carbonylcompounds
TRPNH
NH2
Indolethylamines(eg serotonine)
RONH
NH
Rʹ NH
N
MeOPictet-Spenglerreaction for example Harmineβ-Carbolines
NH
NHH
O
MeO2C
MeO2C
OH
H
H
Rʹ derived from secologanin
Strictosidine aglycone
NH
N
OH
H
H
Ajmalicine
N
N
O
O
H
H
H
Strychnine (C lost)Corynanthe type
Aspidosperma type Iboga type
N
N
OH
OMe
Quinine (C lost)
N
NH CO2Me
Catharanthine
NH
N
CO2MeH
OAc
OH
Vindoline
Complextransformations
NH
NMe
HN
MeN
H
H
Chimonanthine
Cyclizationdimerization
Prenylationcyclization
NH
NMeHO2C
H
Lysergic acid
H
H
Ergot alkaloidsPyrroloindoles Indole monoterpene
1C lost
1C lostFrom AcCoA
SCHEME 18 Tryptophan‐derived alkaloid biosynthetic pathways (gray parts monoterpenic units)
10 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
different structure strychnine is related to the Corynanthe alkaloids incorporating two carbons from acetyl‐CoA Highly modified monoterpene skeletons are derived from the Corynanthe core through CC bond breaking and reorshyganization leading to Iboga‐type (eg catharanthine) and Aspidosperma‐type (eg vindoline) alkaloids The antishycancer drug vinblastine is a heterodimer resulting from the nucleophilic attack of vindoline on a mannich acceptor resulting from catharanthine found in madagascar perishywinkle (Catharanthus roseus) The heteroaromatic comshypounds ellipticine camptothecin and quinine are also derived from a Corynanthe‐type precursor although in this case the biosynthetic relationship may not be obvious due to deep modifications of the skeleton
Finally two important classes of compounds have to be mentioned since they have inspired many synthetic chemists The pyrroloindole alkaloids result from the cyclization of tryptamine as found in physostigmine (formed by a cationic mechanism after methylation in position 3 of the indole not shown) or in chimonanthine (presumably formed by a radical coupling mechanism Scheme 18) The ergot alkaloids are derived from the 33‐dimethylallylation on position 4 of the indole in trypshytophan whose further cyclization and oxidation processes afford the natural products (eg lysergic acid Scheme 18 and ergotamine) which have had important medical applications [34]
NRPS Metabolites and Peptides NRPS enzymes assemble amino acids including nonproteinogenic ones into oligopeptides The enzymes contain several modules and especially an adenylation domain (A) which specifically selects and activates the amino acid to be transferred as a thioester on the nearby peptidyl carrier protein (PCP) [2] A condensation module (C) then catalyzes the formation of the peptide bonds between the newly introduced amino acyl‐PCP (bearing a free amine) and the elongated peptidyl‐PCP thioester At the end of the elongation a cyclization can occur into cyclopeptides but the peptide can also be
transferred to auxiliary enzymes like methyltransferases glycosyltransferases or oxidases (vancomycins are typical products of such functionalizations) [35 36] The formation of heterocycles is also frequently encountered in this metabolism as in penicillins that are derived from the tripeptide α‐aminoadipoyl‐cysteinyl‐valine or telomestatin (Fig 14) [2]
Other Alkaloid Origins There are many other nitrogen sources involved in alkaloid biosyntheses for example nicotinic acid (originated from aspartic acid and intershymediate in nicotine and anabasine biosyntheses) and anthranilic acid (originated from tryptophan and intermedishyate toward acridines or aurachins) The amination reaction (eg through transamination of carbonyl compounds) is also a way to introduce nitrogens in natural products for example from fatty acids steroids (toward Solanum alkaloids or cyclopamines) or other terpenoids (aconitine and atisine have diterpene skeletons while Daphniphyllum alkaloids are triterpene derivatives) Finally nucleic acids can also be precursors of alkaloids like the well‐known caffeine
12 FROM BIOSYNTHESIS TO TOTAL SYNTHESIS STRATEGIES TOWARD THE NATURAL PRODUCT CHEMICAL SPACE
121 The Chemical Space of Natural Products
Natural products occupy an important place in human com munities as demonstrated by their vast use from ancient times to nowadays like dyes fibers oils pershyfumes agrochemicals or drugs Broadly both primary and secondary metabolites could be classified as natural products while the latter as discussed previously are usushyally regarded as the ldquonatural productsrdquo owing to their comshyplexity and diversity arising from a variety of biosynthetic pathways The structural chemical diversity found among
N
S
CO2HO
HNPhH2C
O
Penicillin G
OH
HN
HN
O
O
ONH2
O
OHO
NHMe
OCl
O
NH
NH
NH
O
ClHO
O
HN
H
H
HOOH
OHHO2C
Vancomycin aglycone
ON
NO
O
NN
O
O
N
N O
Me
S
N N
OMeH
Telomestatin
L-AAA-L-CYS-D-VAL
Enzymes
FIGURE 14 Structural diversity of nonribosomal peptide compounds (AAA α‐aminoadipic acid)
FROm BIOSyNTHESIS TO TOTAL SyNTHESIS STRATEGIES TOWARD THE NATuRAL PRODuCT CHEmICAL SPACE 11
all living organisms defining the chemical space of natural products [37] is the consequence of their evolution occurshyring as an adaptation of organisms to their environment It is commonly believed that secondary metabolites are proshyduced as messengers by living organisms or as weapons against enemies and thus they should have certain biological activities in a medicinal point of view [38] Indeed natural products are regarded as one of the main sources of medicines (Fig 15) From the traditional medicinal extracts to every single bioactive molecule the methods of extraction purification identification and biological investigation of natural products have been well established Their complex structures and interesting properties have attracted synthetic chemists to accomplish their total syntheses and that of medicinally relevant anashylogs sometimes in the industrial context [39] Thus tarshygeting the chemical space of natural products has never been more relevant than today Although the discovery of natural products demands time and labor‐consuming manipulations it is worth to notice that the knowledge on this chemical space is still continually growing while biological advances allow for discovering and undershystanding potential targets However increasing the chemical space of human‐made compounds based on natural products should benefit from transdisciplinary colshylaborations such as the use of coupled biosynthetic and chemical synthetic methods to design original ldquounnatural natural productsrdquo [40]
122 The Biosynthetic Pathways as an Inspiration for Synthetic Challenges
1221 Precursor‐Directed Biosyntheses and Mutashysynthetic Strategies to Increase the Chemical Space of Natural Products As the genetics and biochemistry of natural product biosynthesis are better understood novel biosynthetic techniques have been developed to study and generate new diversity in natural product analogs (Scheme 19) Precursor‐directed biosynthesis (PDB) is considered as the earliest example of combining chemical and biological methods for the generation of complex natural product analogs [41 42] This approach compared with the biosynthetic pathway of wild‐type metabolites (Scheme 19a) involves the feeding of analogs of the natural biosynthetic building blocks to the living organisms (Scheme 19b) usually bacteria or fungi which incorporate the modified precursors into the biosynthesized compound mutasynthesis also termed as mutational biosynthesis (mBS) involves the inactivation of a key step of the bioshysynthesis in a mutant microorganism (Scheme 19c) which can then be fed by various modified or advanced building blocks (mutasynthons Scheme 19d) [43] These mutasshyynthons could not be incorporated by the wild type due to specificities of the enzymatic machinery Build up on PDB mBS eliminates the natural biosynthetic intermediate thus generating a less complex mixture of metabolites and making the purification or yield of target products better
O NH
OHO
O
OO
O
O
OH
HOO
O O
OH
HOO
NH ONH2
O
ONH
NH
O
OCl Cl
O
O
OH
HN
HN
HN
HN
OO
HO
HN
OH
OHOH
O
O
HO
HO
OHO
O
OHH2N
O
OH3C
H
O
H
CH3
CH3
H
O O
NO
H
O
HO
O
NO
O
NH
HN
OHO
HONH
ON
H
HO
O
H2N
O OH
ONH
OH
ON OHO
OH
HO OS OH
OOOH
OHO
O
ON
OO
O
HO
O
O OH
OO
Micafunginantifungal
Rapamycinimmunosuppressive
Galantamineanti-Alzheimer
Taxolanticancer
Artemisininantimalarial
Vancomycinantibiotic
FIGURE 15 Some famous natural products currently used as drugs
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product
From Biosynthesis to total synthesis
From Biosynthesis to total synthesis
strategies and tactics for natural Products
Edited by
alexandros l ZograFosAristotle University of Thessaloniki Greece
Copyright copy 2016 by John Wiley amp Sons Inc All rights reserved
Published by John Wiley amp Sons Inc Hoboken New JerseyPublished simultaneously in Canada
No part of this publication may be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechanical photocopying recording scanning or otherwise except as permitted under Section 107 or 108 of the 1976 United States Copyright Act without either the prior written permission of the Publisher or authorization through payment of the appropriate per‐copy fee to the Copyright Clearance Center Inc 222 Rosewood Drive Danvers MA 01923 (978) 750‐8400 fax (978) 750‐4470 or on the web at wwwcopyrightcom Requests to the Publisher for permission should be addressed to the Permissions Department John Wiley amp Sons Inc 111 River Street Hoboken NJ 07030 (201) 748‐6011 fax (201) 748‐6008 or online at httpwwwwileycomgopermissions
Limit of LiabilityDisclaimer of Warranty While the publisher and author have used their best efforts in preparing this book they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose No warranty may be created or extended by sales representatives or written sales materials The advice and strategies contained herein may not be suitable for your situation You should consult with a professional where appropriate Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages including but not limited to special incidental consequential or other damages
For general information on our other products and services or for technical support please contact our Customer Care Department within the United States at (800) 762‐2974 outside the United States at (317) 572‐3993 or fax (317) 572‐4002
Wiley also publishes its books in a variety of electronic formats Some content that appears in print may not be available in electronic formats For more information about Wiley products visit our web site at wwwwileycom
Library of Congress Cataloging‐in‐Publication Data
Names Zografos Alexandros L editorTitle From biosynthesis to total synthesis strategies and tactics for natural products edited by Alexandros L ZografosDescription Hoboken New Jersey John Wiley amp Sons Inc [2016] | Includes bibliographical references and indexIdentifiers LCCN 2015037375 (print) | LCCN 2015047240 (ebook) | ISBN 9781118751732 (cloth) | ISBN 9781118753569 (Adobe PDF) | ISBN 9781118753637 (ePub)Subjects LCSH Organic compoundsndashSynthesis | BiosynthesisClassification LCC QD262 F76 2016 (print) | LCC QD262 (ebook) | DDC 57245ndashdc23LC record available at httplccnlocgov2015037375
Set in 1012pt Times by SPi Global Pondicherry India
Printed in the United States of America
10 9 8 7 6 5 4 3 2 1
Dedicated to my mother father and wife
LIST OF CONTRIBUTORS xiii
PREFACE xv
1 From Biosyntheses to Total Syntheses An Introduction 1Bastien Nay and Xu‐Wen Li
11 From Primary to Secondary Metabolism the Key Building Blocks 1111 Definitions 1112 Energy Supply and Carbon Storing at the Early Stage
of Metabolisms 1113 Glucose as a Starting Material toward Key Building Blocks
of the Secondary Metabolism 1114 Reactions Involved in the Construction of Secondary Metabolites 3115 Secondary Metabolisms 4
12 From Biosynthesis to total Synthesis Strategies toward the Natural Product Chemical Space 10121 the Chemical Space of Natural Products 10122 the Biosynthetic Pathways as an Inspiration
for Synthetic Challenges 11123 the Science of total Synthesis 14124 Conclusion a Journey in the Future of total Synthesis 16
References 16
SECTION I ACETATE BIOSYNTHETIC PATHWAY 19
2 Polyketides 21Franccediloise Schaefers Tobias A M Gulder Cyril Bressy Michael Smietana Erica Benedetti Stellios Arseniyadis Markus Kalesse and Martin Cordes
21 Polyketide Biosynthesis 21211 Introduction 21212 assembly of acetateMalonate‐Derived Metabolites 23213 Classification of Polyketide Biosynthetic Machineries 23214 Conclusion 39
References 40
CONTENTS
viii CoNtENtS
22 Synthesis of Polyketides 44221 asymmetric alkylation Reactions 44222 applications of asymmetric alkylation Reactions in total Synthesis
of Polyketides and Macrolides 60References 8323 Synthesis of Polyketides‐Focus on Macrolides 87
231 Introduction 87232 Stereoselective Synthesis of 13‐Diols asymmetric aldol Reactions 88233 Stereoselective Synthesis of 13‐Diols asymmetric Reductions 106234 application of Stereoselective Synthesis of 13‐Diols in
the total Synthesis of Macrolides 117235 Conclusion 126
References 126
3 Fatty Acids and Their Derivatives 130Anders Vik and Trond Vidar Hansen
31 Introduction 13032 Biosynthesis 130
321 Fatty acids and Lipids 130322 Polyunsaturated Fatty acids 134323 Mediated oxidations of ω‐3 and ω‐6 Polyunsaturated
Fatty acids 13533 Synthesis of ω‐3 and ω‐6 all‐Z Polyunsaturated Fatty acids 140
331 Synthesis of Polyunsaturated Fatty acids by the Wittig Reaction or by the Polyyne Semihydrogenation 140
332 Synthesis of Polyunsaturated Fatty acids via Cross Coupling Reactions 143
34 applications in total Synthesis of Polyunsaturated Fatty acids 145341 Palladium‐Catalyzed Cross Coupling Reactions 145342 Biomimetic transformations of Polyunsaturated Fatty acids 149343 Landmark total Syntheses 153344 Synthesis of Leukotriene B
5 158
35 Conclusion 160acknowledgments 160References 160
4 Polyethers 162Youwei Xie and Paul E Floreancig
41 Introduction 16242 Biosynthesis 162
421 Ionophore antibiotics 162422 Marine Ladder toxins 165423 annonaceous acetogenins and terpene Polyethers 165
43 Epoxide Reactivity and Stereoselective Synthesis 166431 Regiocontrol in Epoxide‐opening Reactions 166432 Stereoselective Epoxide Synthesis 172
44 applications to total Synthesis 176441 acid‐Mediated transformations 176442 Cascades via Epoxonium Ion Formation 179443 Cyclizations under Basic Conditions 181444 Cyclization in Water 182
45 Conclusions 183References 184
CoNtENtS ix
SECTION II MEVALONATE BIOSYNTHETIC PATHWAY 187
5 From Acetate to Mevalonate and Deoxyxylulose Phosphate Biosynthetic Pathways An Introduction to Terpenoids 189Alexandros L Zografos and Elissavet E Anagnostaki
51 Introduction 18952 Mevalonic acid Pathway 19153 Mevalonate‐Independent Pathway 19254 Conclusion 194References 194
6 Monoterpenes and Iridoids 196Mario Waser and Uwe Rinner
61 Introduction 19662 Biosynthesis 196
621 acyclic Monoterpenes 197622 Cyclic Monoterpenes 197623 Iridoids 200624 Irregular Monoterpenes 202
63 asymmetric organocatalysis 203631 Introduction and Historical Background 204632 Enamine Iminium and Singly occupied Molecular
orbital activation 207633 Chiral (Broslashnsted) acids and H‐Bonding Donors 213634 Chiral BroslashnstedLewis Bases and Nucleophilic Catalysis 218635 asymmetric Phase‐transfer Catalysis 220
64 organocatalysis in the total Synthesis of Iridoids and Monoterpenoid Indole alkaloids 225641 (+)‐Geniposide and 7‐Deoxyloganin 226642 (ndash)‐Brasoside and (ndash)‐Littoralisone 227643 (+)‐Mitsugashiwalactone 229644 alstoscholarine 229645 (+)‐aspidospermidine and (+)‐Vincadifformine 230646 (+)‐Yohimbine 230
65 Conclusion 231References 231
7 Sesquiterpenes 236Alexandros L Zografos and Elissavet E Anagnostaki
71 Biosynthesis 23672 Cycloisomerization Reactions in organic Synthesis 244
721 Enyne Cycloisomerization 245722 Diene Cycloisomerization 257
73 application of Cycloisomerizations in the total Synthesis of Sesquiterpenoids 266731 Picrotoxane Sesquiterpenes 266732 aromadendrane Sesquiterpenes Epiglobulol 267733 CubebolndashCubebenes Sesquiterpenes 267734 Ventricos‐7(13)‐ene 270735 Englerins 271736 Echinopines 271737 Cyperolone 273
x CoNtENtS
738 Diverse Sesquiterpenoids 27674 Conclusion 276References 276
8 Diterpenes 279Louis Barriault
81 Introduction 27982 Biosynthesis of Diterpenes Based on Cationic Cyclizations
12‐Shifts and transannular Processes 27983 Pericyclic Reactions and their application in the Synthesis
of Selected Diterpenoids 284831 Dielsndashalder Reaction and Its application in the total
Synthesis of Diterpenes 284832 Cascade Pericyclic Reactions and their application in the total
Synthesis of Diterpenes 29184 Conclusion 293
References 294
9 Higher Terpenes and Steroids 296Kazuaki Ishihara
91 Introduction 29692 Biosynthesis 29693 Cascade Polyene Cyclizations 303
931 Diastereoselective Polyene Cyclizations 303932 ldquoChiral proton (H+)rdquo‐Induced Polyene Cyclizations 304933 ldquoChiral Metal Ionrdquo‐Induced Polyene Cyclizations 308934 ldquoChiral Halonium Ion (X+)rdquo‐Induced Polyene Cyclizations 313935 ldquoChiral Carbocationrdquo‐Induced Polyene Cyclizations 319936 Stereoselective Cyclizations of Homo(polyprenyl)arene
analogs 31994 Biomimetic total Synthesis of terpenes and Steroids through
Polyene Cyclization 31995 Conclusion 328
References 328
SECTION III SHIKIMIC ACID BIOSYNTHETIC PATHWAY 331
10 Lignans Lignins and Resveratrols 333Yu Peng
101 Biosynthesis 3331011 Primary Metabolism of Shikimic acid and aromatic
amino acids 3331012 Lignans and Lignin 335
102 auxiliary‐assisted C(sp3)ndashH arylation Reactions in organic Synthesis 336
103 FriedelndashCrafts Reactions in organic Synthesis 344104 total Synthesis of Lignans by C(sp3)H arylation Reactions 353105 total Synthesis of Lignans and Polymeric Resveratrol by
FriedelndashCrafts Reactions 357106 Conclusion 375References 375
CoNtENtS xi
SECTION IV MIXED BIOSYNTHETIC PATHWAYSndash THE STORY OF ALKALOIDS 381
11 Ornithine and Lysine Alkaloids 383Sebastian Brauch Wouter S Veldmate and Floris P J T Rutjes
111 Biosynthesis of l‐ornithine and l‐Lysine alkaloids 3831111 Biosynthetic Formation of alkaloids
Derived from l‐ornithine 3831112 Biosynthetic Formation of alkaloids
Derived from l‐Lysine 388112 the asymmetric Mannich Reaction in organic Synthesis 392
1121 Chiral amines as Catalysts in asymmetric Mannich Reactions 3941122 Chiral Broslashnsted Bases as Catalysts in asymmetric
Mannich Reactions 3981123 Chiral Broslashnsted acids as Catalysts in asymmetric
Mannich Reactions 4041124 organometallic Catalysts in asymmetric Mannich Reactions 4081125 Biocatalytic asymmetric Mannich Reactions 413
113 Mannich and Related Reactions in the total Synthesis of l‐Lysine‐ and l‐ornithine‐Derived alkaloids 414
114 Conclusion 426References 427
12 Tyrosine Alkaloids 431Uwe Rinner and Mario Waser
121 Introduction 431122 Biosynthesis of tyrosine‐Derived alkaloids 431
1221 Phenylethylamines 4311222 Simple tetrahydroisoquinoline alkaloids 4331223 Modified Benzyltetrahydroisoquinoline alkaloids 4331224 Phenethylisoquinoline alkaloids 4361225 amaryllidaceae alkaloids 4381226 Biosynthetic overview of tyrosine‐Derived alkaloids 442
123 arylndasharyl Coupling Reactions 4421231 Copper‐Mediated arylndasharyl Bond Forming Reactions 4431232 Nickel‐Mediated arylndasharyl Bond Forming Reactions 4461233 Palladium‐Mediated arylndasharyl Bond Forming Reactions 4471234 transition Metal‐Catalyzed Couplings of Nonactivated
aryl Compounds 450124 Synthesis of tyrosine‐Derived alkaloids 456
1241 Synthesis of Modified Benzyltetrahydroisoquinoline alkaloids 4561242 Synthesis of Phenethylisoquinoline alkaloids 4601243 Synthesis of amaryllidaceae alkaloids 462
125 Conclusion 468References 469
13 Histidine and Histidine‐Like Alkaloids 473Ian S Young
131 Introduction 473132 Biosynthesis 473133 atom Economy and Protecting‐Group‐Free Chemistry 480
xii CoNtENtS
134 Challenging the Boundaries of Synthesis PIas 488135 Conclusion 497References 499
14 Anthranilic AcidndashTryptophan Alkaloids 502Zhen‐Yu Tang
141 Biosynthesis 502142 Divergent SynthesisndashCollective total Synthesis 508143 Collective total Synthesis of tryptophan‐Derived alkaloids 510
1431 Monoterpene Indole alkaloids 5101432 Bisindole alkaloids 512
References 517
15 Future Directions of Modern Organic Synthesis 519Jakob Pletz and Rolf Breinbauer
151 Introduction 519152 Enzymes in organic Synthesis Merging total
Synthesis with Biosynthesis 520153 Engineered Biosynthesis 526154 Diversity‐oriented Synthesis Biology‐oriented Synthesis
and Diverted total Synthesis 5331541 Diversity‐oriented Synthesis 5351542 Biology‐oriented Synthesis 5361543 Diverted total Synthesis 539
155 Conclusion 541References 545
INDEX 548
Elissavet E Anagnostaki Department of Chemistry Laboratory of Organic Chemistry Aristotle University of Thessaloniki Thessaloniki Greece and Research and Development Department Pharmathen SA Thessaloniki Greece
Stellios Arseniyadis School of Biological and Chemical Sciences Queen Mary University of London London United Kingdom
Louis Barriault Department of Chemistry University of Ottawa Ottawa Ontario Canada
Erica Benedetti Laboratoire de Chimie et Biochimie et Pharmacologiques et Toxicologiques CNRS-Universiteacute Paris Descartes Faculteacute des Sciences Fondamentales et Biomeacutedicales Paris France
Sebastian Brauch Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
Rolf Breinbauer Institute of Organic Chemistry Technische Universitaumlt Graz Graz Austria
Cyril Bressy Aix Marseille Universiteacute Centrale Marseille CNRS Marseille France
Martin Cordes Institute for Organic Chemistry and Center of Biomolecular Drug Research (BMWZ) Leibniz Universitaumlt Hannover Hannover Germany and Helmholtz Center for Infection Research (HZI) Hannover Germany
Paul E Floreancig Department of Chemistry Chevron Science Center University of Pittsburgh Pittsburgh PA USA
Tobias A M Gulder Department of Chemistry and Center for Integrated Protein Science Munich (CIPSM) Biosystems Chemistry Technische Universitaumlt Muumlnchen Munich Germany
Trond Vidar Hansen School of Pharmacy University of Oslo Oslo Norway
Kazuaki Ishihara Department of Biotechnology Graduate School of Engineering Nagoya University Nagoya Japan
Markus Kalesse Institute for Organic Chemistry and Center of Biomolecular Drug Research (BMWZ) Leibniz Universitaumlt Hannover Hannover Germany and Helmholtz Center for Infection Research (HZI) Hannover Germany
Xu‐Wen Li Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
Bastien Nay Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France
Yu Peng State Key Laboratory of Applied Organic Chemistry Lanzhou University Lanzhou China
Jakob Pletz Institute of Organic Chemistry Technische Universitaumlt Graz Graz Austria
Uwe Rinner Institute of Organic Chemistry Johannes Kepler University Linz Linz Austria and Department of Chemistry College of Science Sultan Qaboos University Muscat Oman
Floris P J T Rutjes Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
LIST oF CoNTRIBUToRS
xiv LIST OF CONTRIBUTORS
Franccediloise Schaefers Department of Chemistry and Center for Integrated Protein Science Munich (CIPSM) Biosystems Chemistry Technische Universitaumlt Muumlnchen Munich Germany
Michael Smietana Institut des Biomoleacutecules Max Mousseron CNRS Universiteacute de Montpellier ENSCM France
Zhen‐Yu Tang Department of Pharmaceutical Engineering College of Chemistry and Chemical Engineering Central South University Changsha China
Wouter S Veldmate Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
Anders Vik School of Pharmacy University of Oslo Oslo Norway
Mario Waser Institute of Organic Chemistry Johannes Kepler University Linz Linz Austria
Youwei Xie Max‐Planck‐Institut fuumlr Kohlenforschung Muumllheim Germany
Ian S Young Bristol‐Myers Squibb Company Chemical Development New Brunswick NJ USA
Alexandros L Zografos Department of Chemistry Laboratory of Organic Chemistry Aristotle University of Thessaloniki Thessaloniki Greece
There is pleasure in the pathless woodsthere is rapture in the lonely shore
there is society where none intrudesby the deep sea and music in its roar
I love not Man the less but Nature moreLord Byron
Preface
The first time I came across with the idea of editing a book that merges selected chapters of biosynthesis and total synthesis was when I was teaching postgraduate courses of natural product synthesis at Aristotle University of Thessaloniki This period I realized that the best way to teach youngsters synthesis was to start from the very origin of inspiration nature and its tools biosynthesis
Over the last decades biosynthesis is filling our gaps of understanding the complex mechanisms of nature and pro-vides useful sources of inspiration not only in the way natural products can be synthesized but also by directing synthetic chemists in developing atom‐economical efficient synthetic methods Several are the examples that mimic biosynthetic guidelines from modern iterative alkylations and aldol reactions to CH oxidations that compile nowadays the modern toolbox of organic synthesis
The handed book is constructed in the logic of presenting the parallel development of biosynthesis and organic meth-odology and how these can be applied in efficient syntheses of natural products The book is divided into four sections each representing the four major biosynthetic pathways of natural products namely acetate mevalonate shikimate biosynthetic pathways and the mixed biosynthetic pathways
of alkaloids These sections are divided into chapters that represent selected classes of natural products for example lipids sesquiterpenoids lignans etc Each of these chapters is further divided into three distinct subchapters (a) biosyn-thesis (b) methodological section and (c) application of the described methodology in the total synthesis of the described family of natural products By this way the readers can be focused in the direct comparison between biosyn-theses and the developed methodologies to construct the crucial for each class of natural product carbon bonds Although the book as it develops is focused on presenting the power of biosynthesis and how this power can be applied in providing inspiration for the efficient synthesis of natural products it was not the authors will to present only biomi-metic total syntheses but rather to exploit the modern synthetic methodologies and recognize their disabilities for further improvement
Of course this book will not have been realized without the excellent work of renowned scientists worldwide working either in the field of biosynthesis or total synthesis who collected the existing knowledge on biosynthesis analyzed the existing modern methodologies and presented a bouquet of selected total syntheses Throughout our
xvi PrEfACE
endeavor to complete this book I learned many things from their expertise but I also realized that only with tight collab-orations you can build long‐lasting friendships I would like to thank them all once again for their trust and effort to complete this book We all hope that the current work will contribute to a better understanding of the current status of
organic chemistry and to the discovery of novel strategies and tactics for the synthesis of natural products
Alexandros L ZografosSeptember 2015
Thessaloniki Greece
From Biosynthesis to Total Synthesis Strategies and Tactics for Natural Products First Edition Edited by Alexandros L Zografoscopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
11 FROM PRIMARY TO SECONDARY METABOLISM THE KEY BUILDING BLOCKS
111 Definitions
The primary and secondary metabolisms are traditionally distinguished by their distribution and utility in the living organism network Primary metabolites include carbohyshydrates lipids nucleic acids and proteins (or their amino acid constituents) and are shared by all living organisms on Earth They are transformed by common pathways which are studied by biochemistry (Fig 11) Secondary metabo-lites are structurally diverse compounds usually produced by a limited number of organisms which synthesize them for a special purpose like defense or signaling through specific biosynthetic pathways They are studied by natural product chemistry This distinction is not always so obvious and some compounds can be studied in the context of both primary and secondary metabolisms This is especially true nowadays with the use of genetic and biomolecular tools which tend to make natural product sciences more and more integrative However an important point to remember is that the primary metabolism furnishes key building blocks to the secondary metabolism It would be difficult to describe in detail the full biosynthetic pathshyways in this section We tried to organize the discussion as a vade mecum synthetically gathering information from extremely useful sources which will be cited at the end of this chapter
112 Energy Supply and Carbon Storing at the Early Stage of Metabolisms
The sunlight is essential to life except in some part of the deep oceans It provides energy for plant photosynthesis that splits molecules of water into protons and electrons and releases O
2 (Scheme 11) A proton gradient inside the plant
chloroplasts then drags a transmembrane ATP synthase comshyplex that produces adenosine triphosphate (ATP) while elecshytrons released from water are transferred to the coenzyme reducer nicotinamide adenine dinucleotide phosphate hydride (NADPH) A major function of chloroplasts is to fix CO
2 as a combination to ribulose‐15‐bisphosphate (RuBP)
performed by RuBP carboxylase (rubisco) forming an instable ldquoC
6rdquo β‐ketoacid This is cleaved into two molecules
of 3‐phosphoglycerate (3‐PGA) which is then reduced into 3‐phosphoglyceraldehyde (3‐PGAL a ldquoC
3rdquo triose phosshy
phate) during the Calvin cycle This is one of the major metabolites in the biosynthesis of carbohydrates like glucose and a biochemical mean for storing and retaining carbon atoms in the living cells
113 Glucose as a Starting Material Toward Key Building Blocks of the Secondary Metabolism
Glucose‐6‐phosphate arises from the phosphorylation of glucose It is the starting material of glycolysis an important process of the primary metabolism which consists in eight enzymatic reactions leading to pyruvic acid (PA)
FROM BIOSYNTHESES TO TOTAL SYNTHESES AN INTRODUCTION
Bastien Nay1 and Xu‐Wen Li2
1
1 Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France2 Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
2 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
(Scheme 12) Important intermediates for the secondary metabolism are produced during glycolysis Glucose glucose‐6‐phosphate and fructose‐6‐phosphate can be converted to other hexoses and pentoses that can be oligoshymerized and enter in the composition of heterosides Additionally fructose‐6‐phosphate connects the pentose
phosphate pathway leading to erythrose‐4‐phosphate toward shikimic acid which is a key metabolite in the biosynthesis of aromatic amino acids (phenylalanine tyrosine or C
6C
3
units) and C6C
1 phenolic compounds The next important
intermediate in glycolysis is 3‐PGAL which can be redishyrected toward methylerythritol‐4‐phosphate (mEP) in the
Primary metabolism Secondary metabolism
The field ofbiochemistry
The field ofnatural product
chemistry
Essential toliving organisms
Essential to theproducer organisms
under particularconditions
Nucleic acids (DNARNA) carbohydrates
lipids amino acidspeptides and proteins
Alkaloids terpenespolyketides
polyphenols andtheir heterosidic form
Biological effects(defense signaling)
Biosyntheticpathways
Main compoundclasses
FIGURE 11 Primary versus secondary metabolisms
PS-I
PS-IIendash
hν H2O H+ and O2
ATP synthase
NADP reductase
H+ (gradient)
H+ADP + Pi ATP
NADPHH+NADP+
H+
Thylakoidcompartment
Chloroplaststroma
Thylakoidmembrane
(a) Light dependent process
(b) Light independent process
CO2
RuBP
Rubisco
Unstable β-ketoacid3-PGA 13-diPGA
ATP
ADP3-PGAL
NADPHH+
NADP+
(Calvin cycle)
trioses tetroses pentoses hexoses (eg glucose) heptoses
Chloroplaststroma
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
CH2OPO32ndash CH2OPO3
2ndash CH2OPO32ndash
CH2OPO32ndash
OOH
HO
HOO
HO
HO2CCO2H
HO
CO2PO32ndash
HO
CHOHO
endash
Cytosol
SCHEME 11 The photosynthetic machinery (PS‐I and PS‐II photosystems I and II)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 3
chloroplast mEP is a starting block in the biosynthesis of terpenes through C
5 isoprene units (isopentenyl diphosphate
(IPP) and dimethylallyl diphosphate (DmAPP)) especially those in C
10 C
20 and C
40 terpenes 3‐PGA is a precursor of
serine and other amino acids while phosphoenolpyruvate (PEP) the precursor of PA is also an intermediate toward the previously mentioned shikimic acid Lastly PA is not only a precursor of the fundamental ldquoC
2rdquo acetyl coenzyme A
(AcCoA) unit but also an intermediate toward aliphatic amino acids and mEP
AcCoA is the building block of fatty acids polyketides and mevalonic acid (mVA) a cytosolic precursor of the C
5 isoprene units for the biosynthesis of terpenes in the
C15
and C30
series (mind it is different from the mEP pathway in product and in cell location) Finally AcCoA enters the citric acid or Krebs cycle which leads to several precursors of amino acids These are oxaloacetic acid precursor of aspartic acid through transamination (thus toward lysine as a nitrogenated C
5N linear unit and methishy
onine as a methyl supplier) and 2‐oxoglutaric acid preshycursor of glutamic acid (and subsequent derivatives such as ornithine as a nitrogenated C
4N linear unit) All these
amino acids are key precursors in the biosynthesis of many alkaloids
114 Reactions Involved in the Construction of Secondary Metabolites
most reactions occurring in the living cells are performed by specialized enzymes which have been classified in an intershynational nomenclature defined by an enzyme commission (EC) number There are six classes of enzymes depending on the biochemical reaction they catalyze EC‐1 oxidoreducshytases (catalyzing oxidoreduction reactions) EC‐2 transfershyases (catalyzing the transfer of functional groups) EC‐3 hydrolases (catalyzing hydrolysis) EC‐4 lyases (breaking bonds through another process than hydrolysis or oxidation leading to a new double bond or a new cycle) EC‐5 isomershyases (catalyzing the isomerization of a molecule) and EC‐6 ligases (forming a covalent bond between two molecules) many subclasses of these enzymes have been described depending on the type of atoms and functional groups involved in the reaction and if any on the cofactor used in this reaction For example several cofactors can be used by dehydrogenases like NAD(P)NAD(P)H FADFADH
2 or
FmNFmNH2 For a description of this classification the
reader can refer to specialized Internet websites like ExplorEnz [1] What is important to realize is that most enzymes are substrate specific and have been selected during
CHOHO
CO2H
O
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
HOOH
HO
HOH2CO
OHC
CH2OPO32ndash
OHHO
CO2H
HO
OH
OH
TYR
PHE
CO2H
NH2
TRP
CH2OPO32ndash
HO
HOH2COH
MEP
C10 C20 and C40 terpenes
CO2HHO
SER
GLY CYS
CO2HOPO3
2ndash
CH2OH
OHHO2C
MVA
C15 and C30 terpenes
Polyketides
Krebscycle
(citric acid)
VAL ALA ILE LEU
CO2H
O
HO2C
CO2HO
CO2H
ASP
LYS
GLU
Glucose-6-phosphate
Fructose-6-phosphate
Erythrose-4-phosphate
3-PGAL
OP2O63ndash OP2O6
3ndash
IPP DMAPP
3-PGA PEP PA
O SCoAAcCoA
3x
Cytosol
Cytosol
Chloroplasts
Chloroplasts
Glycolysis
C2
C5
Shikimic acid Oxaloaceticacid
2-Oxoglutaricacid
MET
THR
PRO
ARGORN
Alkaloids Alkaloids Alkaloids Alkaloids
Aminoacids
Peptidesproteins
Phenolics
C1
C5N C4N
ldquoCH3rdquo
(Indole)C2NC6C3C6C2N C6C1
Pentosephosphatepathway
SCHEME 12 The building block chart involving glycolysis and the Krebs cycle
4 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
evolution to perform specific transformations making natural products with often and yet unknown functions
Secondary metabolites arise from specific biosynthetic pathways which use the previously defined building blocks The bunch of organic reactions involved in these biosynshytheses allows the construction of natural product frameshyworks which are finally diversified through ldquodecorationrdquo steps (Scheme 13) It is not the purpose of this introductive chapter to describe in detail all biosynthetic pathways and the reader can refer to excellent books and articles which have been published elsewhere [2 3]
The reactions involved in the construction of natural product skeletons will be described later for representative classes of compounds The identification of the building block footprint in the natural product skeleton will be emphasized as much as possible sometimes referring to biogenetic speculations [4] After the framework construction the decoration steps will involve as diverse reactions as aliphatic CH oxidations (eg involving a cytochrome P
450 oxygenase) occasionally triggering a
rearrangement heteroatom alkylations (eg methylation by S‐adenosylmethionine) or allylation (by DmAPP) esterifications heteroatom or C‐glycosylations (leading to heterosides) radical couplings (especially for phenols) alcohol oxidations or ketone reductions amineketone transaminations alkene dihydroxylations or epoxidations oxidative halogenations BaeyerndashVilliger oxidations and further oxygenation steps At the end of the biosynthesis such transformations may totally hide the primary building block origin of natural products
115 Secondary Metabolisms
1151 Polyketides Polyketides (or polyacetates) are issued from the oligomerization of C
2 acetate units performed
by polyketide synthases (PKS) and leading to (C2)
n linear
intermediates [5 6] If the (C2)
n intermediates arise from
successive Claisen reactions performed by ketosynthase
domains (KS in nonreducing PKS) a highly reactive poly‐ β‐ketoacyl intermediate H(CH
2CO)
nOH is formed
leading to phenolic and aromatic products through further intramolecular Claisen condensations Furthermore highly reducing PKSs are made of specialized enzymatic subunits working in line or iteratively to functionalize each C
2 linker
bond as CH(OH)CH2 (by ketoreductases (KR)) then as
HCCH (by dehydratases (DH)) and as CH2CH
2 (by enoyl
reductases (ER)) leading to a high degree of functionalization of the final product (Fig 12) By these iterative sequences highly reduced polyketides which can be either linear macrocyclized or polycyclized depending on the reactivity of the biosynthetic intermediates can be formed [7] With the same logic fatty acids are also biosynthesized by fatty acid synthases
moreover the PKS enzyme can be hybridized with nonshyribosomal peptide synthetase (NRPS) domains (see also ldquoNRPS metabolites and peptidesrdquo in the ldquoAlkaloidsrdquo secshytion) leading to the acylation of an amino acid by the (C
2)
n
acyl intermediate As previously the functionalization of the acyl chain depends on the PKS enzyme and the PKSNRPS products are also extremely diversified (eg hirsutellone B Fig 12) [8]
1152 Terpenes Terpenes are derived from the oligoshymerization of the C
5 isoprene units DmAPP and IPP Both
precursors are prompt to generate either an allylic cation (the diphosphate is a good leaving group) or a tertiary carbocation respectively which makes the IPP easy to react with DmAPP (Scheme 14) This reaction happens in the active site of a terpene synthase which activates the deparshyture of the diphosphate group from DmAPP thanks to Lewis acid activation (a metal like mg2+ mn2+ or Co2+ is present in the enzyme active site [9]) This leads to geranyl (C
10
monoterpene precursor) or farnesyl (C15
sesquiterpene precursor) diphosphate depending on the location of the enzyme (chloroplast for the mEP pathway or cytosol for the mVA pathway) Geranylgeranyl (C
20 diterpene) and
Constructionreactions
Decoration reaction(functionalization)Building
blocksNatural product
frameworksNatural products
HH
OH OAcH
HO O OH
OHO
OBz
P450
P450
P450
P450
P450 P450 Oxidativeauxiliaryenzymes
Taxadiene
OP2O63ndash
OP2O63ndash
3times
(a)
(b)
10-Deacetylbaccatin III
DMAPP
IPPand electrophilic
cyclizations
SCHEME 13 (a) From building blocks to natural products and (b) the example of 10‐deacetylbaccatin III
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 5
farnesylfarnesyl (C30
triterpene precursor) diphosphates can also be obtained by further additions of IPP leading to longer linear intermediates
The cyclization of linear precursors is achieved by speshycialized cyclases which generate a poorly functionalized natural product framework [10 11] Auxiliary enzymes such as oxygenases then increase the complexity and the diversity
of compounds by further functionalization (Scheme 13b) [12] A high degree of oxidation can be observed in comshypounds like thapsigargin paclitaxel or bilobalide (Fig 13) The biosynthesis of this last compound for example involves a high oxygenation pattern two Wagnerndashmeerwein rearrangements and several oxidative cleavages leading to the loss of five carbons The resulting natural products can
KS
S-ACP
O O
KR
S-ACP
OH O
S-MAT
O
YesNo
R
R
R
YesNoDH
S-ACP
OR
YesNoER
S-ACP
OR
YesNoTE
OH
OR
New cycle
New cycleor
release
New cycleor
release
New cycleor
release
Release
R in
crem
ente
d by
two
carb
ons
HO
O
S-ACP
O
Claisen condensation (ndashCO2)
reduction (NADPH)
dehydration (ndashH2O)
+
reduction (NADPH)
hydrolysis (H2O)
O O NH
OH
OH
H H
H H
Hirsutellone B(mixed PKSNRPS
product)
OO
O
OHOH
HO OH
Norsolorinic acid
O
O
O
O CHO
OH
HH
H
H
HH
OHHemibrevetoxin B
OOHO
OHO
HOHO OH
Erythronolide A
OH
OH
HO
Panaxytriol
From ahexanoylstartingblock
H
H
Fromtyrosine
H
1C lost fromdecarboxylation
FIGURE 12 Chemical logic of polyketide construction leading to variable functionalization of the elongated acyl chain and examples of resulting chemical diversity ACP acyl carrier protein DH dehydratase ER enoyl reductase KR ketoreductase KS ketosynthase mAT malonyl acyl transferase TE thioesterase
DMAPPOP2O6
3ndashOP2O6
3ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
Geranyl diphosphate (GPP)2 times IPP
H H
Geranylgeranyl diphosphate (GGPP)
Examplesverticillane
casbanetaxane
phorbol
Exampleslabdane
pimaranekauraneabietane
aphidicolanegibberellane
IPP
SCHEME 14 Early assembly of C5 units in terpene biosynthesis leading to diterpenes (C
20)
6 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
thus be extremely modified with structures whose biogenetic origin is far from being obvious at first sight and cannot be determined without further experiments such as isotopic labeling
1153 Flavonoids Resveratrols Gallic Acids and Further Polyphenolics We have previously discussed the polyketide origin of some phenolic compounds based on the (C
2)
n motif Other polyphenols like gallic acids are directly
derived by the aromatization of shikimic acid (C6C
1 building
block Scheme 12) [13] The C6C
3 building blocks are availshy
able from the conversion of phenylalanine and tyrosine into cinnamic and p‐coumaric acids respectively and then by further hydroxylation steps (Scheme 15) These can dimerize into lignans (eg podophyllotoxin) [14 15] through radical processes or converted to low molecular weight compounds like eugenol coumarins or vanillin [16] The coenzyme A thioesters of these C
6C
3 acids can be used as initiator units
by specialized ketosynthases for an elongation by two acetyl units leading to aromatic polyketides like styrylpyrones or diarylheptanoids (eg curcumin) [17] Important comshypounds from this metabolism are flavonoids (C
6C
3C
6) [18]
and stilbenoids (C6C
2C
6) (a decarboxylation occurs during
the aryl cyclization) [19] which are synthesized by chalcone synthase and stilbene synthase respectively Flavonoids (eg catechin) and stilbenes (eg resveratrol) are present in large amounts in fruits and vegetables and may exert their radical scavenging properties in vivo
1154 Alkaloids Alkaloids are nitrogen‐containing compounds The nitrogen(s) can be involved in an amine function conferring basicity to the natural product (like ldquoalkalirdquo) or in less or nonbasic functions such as an amide a nitrile an isonitrile or an ammonium salt (quaternary amines) For amines protonation often occurs at physiological pH and may condition their biological activity In many cases the nitrogen is biogenetically derived from an amino acid We will thus discuss alkaloids according to their amino acid origin
Alkaloids Derived from the Krebs Cycle (Lysine and Ornithine Derived) As shown previously (Scheme 12) the Krebs cycle is a crucial metabolic process which leads to α‐ketoacids (oxaloacetic and 2‐oxoglutaric acids) Their enzymatic transamination affords the two amino acidsmdashaspartic acid and glutamic acidmdashwhich are the direct biosynthetic precursors of amino acids lysine and ornithine respectively These in turn produce cadaverine a ldquoC
5Nrdquo unit
and putrescine a ldquoC4Nrdquo unit which are major components
for the biosynthesis of important alkaloids as will be discussed later (Scheme 16) Additionally ornithine is a precursor of arginine another important amino acid
ornithine‐derived alkaloids (incorporating the c4n
unit) Putrescine is derived from the decarboxylation of ornithine and is a precursor of linear polyamines like spermshyine After enzymatic methylation of one amine of putrescine
C10 C15
C15
C10
C10
C30
CO2H
Chrysanthemic acid
OH
Menthol
O
HO
H
HOGlc
MeO2CLoganin (iridoid)
H
H
H
HH
OO
OParthenolide
O
O
OHOHO
OAcHO
OnC3H7
O
O
O
nC7H15
Thapsigargin
O
O
OO
H
H
O
Cleavage
H
H
Artemisinin
C20 C40
O
O
OO
OO
OHOHH
Bilobalide(highly rearrangedand missing 5 C)
CleavageH
Highoxidativecleavages
HO OAcH
AcO O OH
OOOBz
O
Ph
BzNH
OHFrom PHE Paclitaxel
C25O
H
HO
OHCH
OH Ophiobolin A
H
HO
H
H
HHCholesterol
(missing 3 C WM)
H
H H H
H
H
Hopene
β-Carotene
C30 - 3
C15
C20 - 5WM
WM
FIGURE 13 Chemical diversity in the terpene series (Wm Wagnerndashmeerwein shifts lost carbons bold bonds are remnant of primary building blocks)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 7
in the presence of S‐adenosylmethionine transamination of the other affords γ‐(N‐methylamino)aldehyde [20] The resulting cyclic iminium is a key intermediate in the formation of many medicinally important alkaloids such as
the plant‐derived compounds cocaine atropine or the calysshytegines [21 22] Indeed this iminium is a mannich acceptor which can react with various nucleophiles the first of those being the carbanion of acetyl‐CoA Thus after a stepwise
CO2H
RCinnamic acid (R=H)p-Coumaric acid (R=OH)
RO
PHETYR
OH
[H] [O]
lignans
OH
O
O
O
O
OMeMeO
OMe
C mdash C andor C mdash Oradical couplings
After E Zisomerization
for example Podophyllotoxin
O ORO
Coumarins(eg scopoletin)
[O]
Prenylationcyclizationcleavage[O]
O OO
Furocoumarins(eg psoralen)
RO
nAcetyl-CoA
thencyclization
n = 2 styrylpyrones
O O
OMe
n = 3 avonoids(from chalcone synthase)
C6C3
H
H H
From AcCoA
O
n = 3 stilbenoids(from stilbene synthase)
HO
OH
OHFrom AcCoA From AcCoA(ndashCO2)
Resveratrol
HO
OH
OH
OH
OH
H
Catechin
MeO Yangonin
From AcCoA
SCHEME 15 The phenylpropanoid biosynthetic pathways
LYS
CO2
Cadaverine
O NH
H2O
NH
Piperideineiminium
Pelletierine
AcAcCoA
CO2
NH
O
N
O
Pseudopelletierine
NH
Ph
OH
Ph
O
Lobeline
NH
δ+
δndash
N
H
H
N
H
OH
Lupinine
N
H N
H
(+)-Sparteine
N
NH
O (ndash)-Cytisine
NH
CO2H
Pipecolic acid
N
OHHO
OH
HO
H
Castanos permine
ORN
CO2
NH2 NH2 NH2 NH2
NH2
NH2
NH2
PutrescineO
NHMe
N-Methylpyrrolinium
2 timesAcCoA
NMe
ARG
n = 1 spermidinen = 2 homospermidine
NMe
O
HygrineCO2
CO2
MeN
O
[O]
TropinoneCO2
HNHO
OHOH
OH
Calystegine B2
MeN
OAtropine
O
Ph
HO
NMe
NMe
NMe
O
Cuscohygrine
HN
HN
NH2
( )n
O
n = 2
N
HHO
Retronecine
HH
H
H
H
HO
H H
H
SCHEME 16 Lysine‐ and ornithine‐derived alkaloid biosynthetic pathways (mind the structural similarities)
8 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
elongation by two AcCoA units either decarboxylation can occur leading to the acetonylpyrrolidine hygrine or a secshyond mannich reaction by the intramolecular attack of the acetoacetate anion onto an oxidation‐derived pyrrolinium leading to the tropane skeleton (tropinone) The acetoacetyl‐CoA intermediate can also react intermolecularly with another pyrrolinium cation leading to cuscohygrine after decarboxylation Finally the pyrrolizidine alkaloids [23] are derived from homospermidine which when submitted to terminal oxidative deamination leads to the bicyclic skelshyeton of retronecine and further Senecio alkaloids We can mention herein that ornithine is a biosynthetic precursor of arginine bearing a guanidine function which is an intermediate toward the toxic compounds tetrodotoxin and saxitoxin (not shown)
lysine‐derived alkaloids (incorporating the c5n
unit) From lysine to piperidine alkaloids the biosynthetic steps parallel the one previously described from ornithine Indeed the oxidative deamination of cadaverine affords a δ‐amino aldehyde which cyclizes through imine formation into piperideine Protonation results in a mannich acceptor which is able to react with various nucleophiles such as β‐ketothioester anions The first product of these reactions is pelletierine which can further react through an intramolecular mannich
reaction leading to pseudopelletierine Quinolizidines [24] can also be formed first from the mannich reaction of the piperideine acceptor with the corresponding enamine nucleoshyphile and then after additional transformation steps leading for example to lupinine sparteine or cytisine
Indolizidine alkaloids [15] such as castanospermine and swainsonine are formed from pipecolic acid an amino acid derived from lysine which can be elongated by malonyl‐CoA followed by ring closure When protonated these alkaloids are oxonium mimics strongly inhibiting glycosidases
Tyrosine‐ and Phenylalanine‐Derived Alkaloids Tyrosine and phenylalanine amino acids are bearing the phenylshyethylamine moiety of many medicinally relevant alkaloids Further hydroxylations on the aromatic carbocycle or on the aliphatic part can be observed methylations can occur on phenolic oxygens and on the amine leading to catecholamines (adrenaline noradrenaline dopamine) Arylethylamines are also usual to react with endogenous aldehydes through PictetndashSpengler reactions [25] leading to important biosynthetic intermediates (Scheme 17) like
bull Reticuline from the reaction with 4‐hydroxyphenylacetshyaldehyde toward benzyltetrahydroisoquinoline alkaloids
NH2
Phenylethylamines
RONH TetrahydroisoquinolinesRO
RʹCHO
Rʹ
NH
(CH2)n
MeO
HO
Benzyltetrahydroisoquinoline(n = 1) for example reticuline
orPhenethyltetrahydroisoquinoline
(n = 2) eg autumnaline
IntermolecularCmdashO phenol
couplings
Curarealkaloids
IntramolecularC mdash C phenol
couplings
ONMe
HHO
H
HO
Morphine
NMe
MeO
HO
MeOOH
Isoboldine
O
O
OMe
NO2
CO2H
Aristolochic acid
OMe
O
NHAcMeO
MeOMeO
Colchicine
N
O
O
OMe
OMeBerberine
IntramolecularCmdash C coupling
and furtheroxidative events
Rʹ derivedfrom PHE
Pictet-Spenglerreaction
TYR
HO
HO CHO
HNHO
HO
OH
OMeO
OH
NMe
[O]
GalantamineNorbelladine
Rʹ derived fromsecologanin
NAc
HO
HO
O
CO2MeH
H
H
OHIpecosideaglycone
Ipecac alkaloids
1ClostCmdash C cleavage
and ring extension
H
ROH
1C lost
Cleavage
SCHEME 17 Tyrosine‐derived alkaloid biosynthetic pathways (double head arrows figure bond cleavages during biosynthetic processes)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 9
morphine berberine tubocurarine isoboldine or the highly modified aristolochic acid [26 27]
bull Automnaline from the reaction with 3‐(4‐hydroxyphenyl)propanal toward phenylethyltetrahydroisoquinoline alkaloids colchicine cephalotaxine or schelhammerishycine [28 29]
bull Ipecoside from the reaction of dopamine with secoloshyganin toward terpene tetrahydroisoquinoline alkaloids ipecoside or emetine
Lastly norbelladine (top of Scheme 17) is issued from the reductive amination of 34‐dihydroxybenzaldehyde (derived from phenylalanine) with tyramine (derived from tyrosine) and constitutes a biosynthetic node leading to Amaryllidaceae alkaloids such as galantamine crinine or lycorine depending on the topology of phenolic couplings In all these biosynthetic routes radical phenolic couplings are key reactions for CC and CO bond formations and rearrangements [30 31]
Tryptophan‐Derived Indole and Indole Monoterpene Alkaloids As for alkaloids derived from tyrosine and phenylalanine those derived from tryptophan are formed after decarboxylation of the amino acid (into tryptamine) and possible hydroxylation of the aromatic carbocycle (eg serotonin) and N‐methylation (eg psilocin) As previously tryptamine can also react through PictetndashSpengler reactions to form tetrahydro‐β‐carbolines which can be aromatized for example into harmine (Scheme 18) [16]
When the aldehyde partner of the PictetndashSpengler reacshytion with tryptamine is the terpene secologanin strictosidine is formed as an entry toward the vast monoterpene indole alkaloids [32 33] Hydrolysis of the glucosidic part releases the strictosidine aglycone bearing an aldehyde while iminshyium formation and further cyclization and reduction can lead to ajmalicine (from oxocyclization) or yohimbine (from carshybocyclization) These alkaloids are referred to as from the Corynanthe type with the monoterpene carbon skeleton unmodified Although it misses one carbon and has a very
RO
Carbonylcompounds
TRPNH
NH2
Indolethylamines(eg serotonine)
RONH
NH
Rʹ NH
N
MeOPictet-Spenglerreaction for example Harmineβ-Carbolines
NH
NHH
O
MeO2C
MeO2C
OH
H
H
Rʹ derived from secologanin
Strictosidine aglycone
NH
N
OH
H
H
Ajmalicine
N
N
O
O
H
H
H
Strychnine (C lost)Corynanthe type
Aspidosperma type Iboga type
N
N
OH
OMe
Quinine (C lost)
N
NH CO2Me
Catharanthine
NH
N
CO2MeH
OAc
OH
Vindoline
Complextransformations
NH
NMe
HN
MeN
H
H
Chimonanthine
Cyclizationdimerization
Prenylationcyclization
NH
NMeHO2C
H
Lysergic acid
H
H
Ergot alkaloidsPyrroloindoles Indole monoterpene
1C lost
1C lostFrom AcCoA
SCHEME 18 Tryptophan‐derived alkaloid biosynthetic pathways (gray parts monoterpenic units)
10 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
different structure strychnine is related to the Corynanthe alkaloids incorporating two carbons from acetyl‐CoA Highly modified monoterpene skeletons are derived from the Corynanthe core through CC bond breaking and reorshyganization leading to Iboga‐type (eg catharanthine) and Aspidosperma‐type (eg vindoline) alkaloids The antishycancer drug vinblastine is a heterodimer resulting from the nucleophilic attack of vindoline on a mannich acceptor resulting from catharanthine found in madagascar perishywinkle (Catharanthus roseus) The heteroaromatic comshypounds ellipticine camptothecin and quinine are also derived from a Corynanthe‐type precursor although in this case the biosynthetic relationship may not be obvious due to deep modifications of the skeleton
Finally two important classes of compounds have to be mentioned since they have inspired many synthetic chemists The pyrroloindole alkaloids result from the cyclization of tryptamine as found in physostigmine (formed by a cationic mechanism after methylation in position 3 of the indole not shown) or in chimonanthine (presumably formed by a radical coupling mechanism Scheme 18) The ergot alkaloids are derived from the 33‐dimethylallylation on position 4 of the indole in trypshytophan whose further cyclization and oxidation processes afford the natural products (eg lysergic acid Scheme 18 and ergotamine) which have had important medical applications [34]
NRPS Metabolites and Peptides NRPS enzymes assemble amino acids including nonproteinogenic ones into oligopeptides The enzymes contain several modules and especially an adenylation domain (A) which specifically selects and activates the amino acid to be transferred as a thioester on the nearby peptidyl carrier protein (PCP) [2] A condensation module (C) then catalyzes the formation of the peptide bonds between the newly introduced amino acyl‐PCP (bearing a free amine) and the elongated peptidyl‐PCP thioester At the end of the elongation a cyclization can occur into cyclopeptides but the peptide can also be
transferred to auxiliary enzymes like methyltransferases glycosyltransferases or oxidases (vancomycins are typical products of such functionalizations) [35 36] The formation of heterocycles is also frequently encountered in this metabolism as in penicillins that are derived from the tripeptide α‐aminoadipoyl‐cysteinyl‐valine or telomestatin (Fig 14) [2]
Other Alkaloid Origins There are many other nitrogen sources involved in alkaloid biosyntheses for example nicotinic acid (originated from aspartic acid and intershymediate in nicotine and anabasine biosyntheses) and anthranilic acid (originated from tryptophan and intermedishyate toward acridines or aurachins) The amination reaction (eg through transamination of carbonyl compounds) is also a way to introduce nitrogens in natural products for example from fatty acids steroids (toward Solanum alkaloids or cyclopamines) or other terpenoids (aconitine and atisine have diterpene skeletons while Daphniphyllum alkaloids are triterpene derivatives) Finally nucleic acids can also be precursors of alkaloids like the well‐known caffeine
12 FROM BIOSYNTHESIS TO TOTAL SYNTHESIS STRATEGIES TOWARD THE NATURAL PRODUCT CHEMICAL SPACE
121 The Chemical Space of Natural Products
Natural products occupy an important place in human com munities as demonstrated by their vast use from ancient times to nowadays like dyes fibers oils pershyfumes agrochemicals or drugs Broadly both primary and secondary metabolites could be classified as natural products while the latter as discussed previously are usushyally regarded as the ldquonatural productsrdquo owing to their comshyplexity and diversity arising from a variety of biosynthetic pathways The structural chemical diversity found among
N
S
CO2HO
HNPhH2C
O
Penicillin G
OH
HN
HN
O
O
ONH2
O
OHO
NHMe
OCl
O
NH
NH
NH
O
ClHO
O
HN
H
H
HOOH
OHHO2C
Vancomycin aglycone
ON
NO
O
NN
O
O
N
N O
Me
S
N N
OMeH
Telomestatin
L-AAA-L-CYS-D-VAL
Enzymes
FIGURE 14 Structural diversity of nonribosomal peptide compounds (AAA α‐aminoadipic acid)
FROm BIOSyNTHESIS TO TOTAL SyNTHESIS STRATEGIES TOWARD THE NATuRAL PRODuCT CHEmICAL SPACE 11
all living organisms defining the chemical space of natural products [37] is the consequence of their evolution occurshyring as an adaptation of organisms to their environment It is commonly believed that secondary metabolites are proshyduced as messengers by living organisms or as weapons against enemies and thus they should have certain biological activities in a medicinal point of view [38] Indeed natural products are regarded as one of the main sources of medicines (Fig 15) From the traditional medicinal extracts to every single bioactive molecule the methods of extraction purification identification and biological investigation of natural products have been well established Their complex structures and interesting properties have attracted synthetic chemists to accomplish their total syntheses and that of medicinally relevant anashylogs sometimes in the industrial context [39] Thus tarshygeting the chemical space of natural products has never been more relevant than today Although the discovery of natural products demands time and labor‐consuming manipulations it is worth to notice that the knowledge on this chemical space is still continually growing while biological advances allow for discovering and undershystanding potential targets However increasing the chemical space of human‐made compounds based on natural products should benefit from transdisciplinary colshylaborations such as the use of coupled biosynthetic and chemical synthetic methods to design original ldquounnatural natural productsrdquo [40]
122 The Biosynthetic Pathways as an Inspiration for Synthetic Challenges
1221 Precursor‐Directed Biosyntheses and Mutashysynthetic Strategies to Increase the Chemical Space of Natural Products As the genetics and biochemistry of natural product biosynthesis are better understood novel biosynthetic techniques have been developed to study and generate new diversity in natural product analogs (Scheme 19) Precursor‐directed biosynthesis (PDB) is considered as the earliest example of combining chemical and biological methods for the generation of complex natural product analogs [41 42] This approach compared with the biosynthetic pathway of wild‐type metabolites (Scheme 19a) involves the feeding of analogs of the natural biosynthetic building blocks to the living organisms (Scheme 19b) usually bacteria or fungi which incorporate the modified precursors into the biosynthesized compound mutasynthesis also termed as mutational biosynthesis (mBS) involves the inactivation of a key step of the bioshysynthesis in a mutant microorganism (Scheme 19c) which can then be fed by various modified or advanced building blocks (mutasynthons Scheme 19d) [43] These mutasshyynthons could not be incorporated by the wild type due to specificities of the enzymatic machinery Build up on PDB mBS eliminates the natural biosynthetic intermediate thus generating a less complex mixture of metabolites and making the purification or yield of target products better
O NH
OHO
O
OO
O
O
OH
HOO
O O
OH
HOO
NH ONH2
O
ONH
NH
O
OCl Cl
O
O
OH
HN
HN
HN
HN
OO
HO
HN
OH
OHOH
O
O
HO
HO
OHO
O
OHH2N
O
OH3C
H
O
H
CH3
CH3
H
O O
NO
H
O
HO
O
NO
O
NH
HN
OHO
HONH
ON
H
HO
O
H2N
O OH
ONH
OH
ON OHO
OH
HO OS OH
OOOH
OHO
O
ON
OO
O
HO
O
O OH
OO
Micafunginantifungal
Rapamycinimmunosuppressive
Galantamineanti-Alzheimer
Taxolanticancer
Artemisininantimalarial
Vancomycinantibiotic
FIGURE 15 Some famous natural products currently used as drugs
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product
From Biosynthesis to total synthesis
strategies and tactics for natural Products
Edited by
alexandros l ZograFosAristotle University of Thessaloniki Greece
Copyright copy 2016 by John Wiley amp Sons Inc All rights reserved
Published by John Wiley amp Sons Inc Hoboken New JerseyPublished simultaneously in Canada
No part of this publication may be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechanical photocopying recording scanning or otherwise except as permitted under Section 107 or 108 of the 1976 United States Copyright Act without either the prior written permission of the Publisher or authorization through payment of the appropriate per‐copy fee to the Copyright Clearance Center Inc 222 Rosewood Drive Danvers MA 01923 (978) 750‐8400 fax (978) 750‐4470 or on the web at wwwcopyrightcom Requests to the Publisher for permission should be addressed to the Permissions Department John Wiley amp Sons Inc 111 River Street Hoboken NJ 07030 (201) 748‐6011 fax (201) 748‐6008 or online at httpwwwwileycomgopermissions
Limit of LiabilityDisclaimer of Warranty While the publisher and author have used their best efforts in preparing this book they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose No warranty may be created or extended by sales representatives or written sales materials The advice and strategies contained herein may not be suitable for your situation You should consult with a professional where appropriate Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages including but not limited to special incidental consequential or other damages
For general information on our other products and services or for technical support please contact our Customer Care Department within the United States at (800) 762‐2974 outside the United States at (317) 572‐3993 or fax (317) 572‐4002
Wiley also publishes its books in a variety of electronic formats Some content that appears in print may not be available in electronic formats For more information about Wiley products visit our web site at wwwwileycom
Library of Congress Cataloging‐in‐Publication Data
Names Zografos Alexandros L editorTitle From biosynthesis to total synthesis strategies and tactics for natural products edited by Alexandros L ZografosDescription Hoboken New Jersey John Wiley amp Sons Inc [2016] | Includes bibliographical references and indexIdentifiers LCCN 2015037375 (print) | LCCN 2015047240 (ebook) | ISBN 9781118751732 (cloth) | ISBN 9781118753569 (Adobe PDF) | ISBN 9781118753637 (ePub)Subjects LCSH Organic compoundsndashSynthesis | BiosynthesisClassification LCC QD262 F76 2016 (print) | LCC QD262 (ebook) | DDC 57245ndashdc23LC record available at httplccnlocgov2015037375
Set in 1012pt Times by SPi Global Pondicherry India
Printed in the United States of America
10 9 8 7 6 5 4 3 2 1
Dedicated to my mother father and wife
LIST OF CONTRIBUTORS xiii
PREFACE xv
1 From Biosyntheses to Total Syntheses An Introduction 1Bastien Nay and Xu‐Wen Li
11 From Primary to Secondary Metabolism the Key Building Blocks 1111 Definitions 1112 Energy Supply and Carbon Storing at the Early Stage
of Metabolisms 1113 Glucose as a Starting Material toward Key Building Blocks
of the Secondary Metabolism 1114 Reactions Involved in the Construction of Secondary Metabolites 3115 Secondary Metabolisms 4
12 From Biosynthesis to total Synthesis Strategies toward the Natural Product Chemical Space 10121 the Chemical Space of Natural Products 10122 the Biosynthetic Pathways as an Inspiration
for Synthetic Challenges 11123 the Science of total Synthesis 14124 Conclusion a Journey in the Future of total Synthesis 16
References 16
SECTION I ACETATE BIOSYNTHETIC PATHWAY 19
2 Polyketides 21Franccediloise Schaefers Tobias A M Gulder Cyril Bressy Michael Smietana Erica Benedetti Stellios Arseniyadis Markus Kalesse and Martin Cordes
21 Polyketide Biosynthesis 21211 Introduction 21212 assembly of acetateMalonate‐Derived Metabolites 23213 Classification of Polyketide Biosynthetic Machineries 23214 Conclusion 39
References 40
CONTENTS
viii CoNtENtS
22 Synthesis of Polyketides 44221 asymmetric alkylation Reactions 44222 applications of asymmetric alkylation Reactions in total Synthesis
of Polyketides and Macrolides 60References 8323 Synthesis of Polyketides‐Focus on Macrolides 87
231 Introduction 87232 Stereoselective Synthesis of 13‐Diols asymmetric aldol Reactions 88233 Stereoselective Synthesis of 13‐Diols asymmetric Reductions 106234 application of Stereoselective Synthesis of 13‐Diols in
the total Synthesis of Macrolides 117235 Conclusion 126
References 126
3 Fatty Acids and Their Derivatives 130Anders Vik and Trond Vidar Hansen
31 Introduction 13032 Biosynthesis 130
321 Fatty acids and Lipids 130322 Polyunsaturated Fatty acids 134323 Mediated oxidations of ω‐3 and ω‐6 Polyunsaturated
Fatty acids 13533 Synthesis of ω‐3 and ω‐6 all‐Z Polyunsaturated Fatty acids 140
331 Synthesis of Polyunsaturated Fatty acids by the Wittig Reaction or by the Polyyne Semihydrogenation 140
332 Synthesis of Polyunsaturated Fatty acids via Cross Coupling Reactions 143
34 applications in total Synthesis of Polyunsaturated Fatty acids 145341 Palladium‐Catalyzed Cross Coupling Reactions 145342 Biomimetic transformations of Polyunsaturated Fatty acids 149343 Landmark total Syntheses 153344 Synthesis of Leukotriene B
5 158
35 Conclusion 160acknowledgments 160References 160
4 Polyethers 162Youwei Xie and Paul E Floreancig
41 Introduction 16242 Biosynthesis 162
421 Ionophore antibiotics 162422 Marine Ladder toxins 165423 annonaceous acetogenins and terpene Polyethers 165
43 Epoxide Reactivity and Stereoselective Synthesis 166431 Regiocontrol in Epoxide‐opening Reactions 166432 Stereoselective Epoxide Synthesis 172
44 applications to total Synthesis 176441 acid‐Mediated transformations 176442 Cascades via Epoxonium Ion Formation 179443 Cyclizations under Basic Conditions 181444 Cyclization in Water 182
45 Conclusions 183References 184
CoNtENtS ix
SECTION II MEVALONATE BIOSYNTHETIC PATHWAY 187
5 From Acetate to Mevalonate and Deoxyxylulose Phosphate Biosynthetic Pathways An Introduction to Terpenoids 189Alexandros L Zografos and Elissavet E Anagnostaki
51 Introduction 18952 Mevalonic acid Pathway 19153 Mevalonate‐Independent Pathway 19254 Conclusion 194References 194
6 Monoterpenes and Iridoids 196Mario Waser and Uwe Rinner
61 Introduction 19662 Biosynthesis 196
621 acyclic Monoterpenes 197622 Cyclic Monoterpenes 197623 Iridoids 200624 Irregular Monoterpenes 202
63 asymmetric organocatalysis 203631 Introduction and Historical Background 204632 Enamine Iminium and Singly occupied Molecular
orbital activation 207633 Chiral (Broslashnsted) acids and H‐Bonding Donors 213634 Chiral BroslashnstedLewis Bases and Nucleophilic Catalysis 218635 asymmetric Phase‐transfer Catalysis 220
64 organocatalysis in the total Synthesis of Iridoids and Monoterpenoid Indole alkaloids 225641 (+)‐Geniposide and 7‐Deoxyloganin 226642 (ndash)‐Brasoside and (ndash)‐Littoralisone 227643 (+)‐Mitsugashiwalactone 229644 alstoscholarine 229645 (+)‐aspidospermidine and (+)‐Vincadifformine 230646 (+)‐Yohimbine 230
65 Conclusion 231References 231
7 Sesquiterpenes 236Alexandros L Zografos and Elissavet E Anagnostaki
71 Biosynthesis 23672 Cycloisomerization Reactions in organic Synthesis 244
721 Enyne Cycloisomerization 245722 Diene Cycloisomerization 257
73 application of Cycloisomerizations in the total Synthesis of Sesquiterpenoids 266731 Picrotoxane Sesquiterpenes 266732 aromadendrane Sesquiterpenes Epiglobulol 267733 CubebolndashCubebenes Sesquiterpenes 267734 Ventricos‐7(13)‐ene 270735 Englerins 271736 Echinopines 271737 Cyperolone 273
x CoNtENtS
738 Diverse Sesquiterpenoids 27674 Conclusion 276References 276
8 Diterpenes 279Louis Barriault
81 Introduction 27982 Biosynthesis of Diterpenes Based on Cationic Cyclizations
12‐Shifts and transannular Processes 27983 Pericyclic Reactions and their application in the Synthesis
of Selected Diterpenoids 284831 Dielsndashalder Reaction and Its application in the total
Synthesis of Diterpenes 284832 Cascade Pericyclic Reactions and their application in the total
Synthesis of Diterpenes 29184 Conclusion 293
References 294
9 Higher Terpenes and Steroids 296Kazuaki Ishihara
91 Introduction 29692 Biosynthesis 29693 Cascade Polyene Cyclizations 303
931 Diastereoselective Polyene Cyclizations 303932 ldquoChiral proton (H+)rdquo‐Induced Polyene Cyclizations 304933 ldquoChiral Metal Ionrdquo‐Induced Polyene Cyclizations 308934 ldquoChiral Halonium Ion (X+)rdquo‐Induced Polyene Cyclizations 313935 ldquoChiral Carbocationrdquo‐Induced Polyene Cyclizations 319936 Stereoselective Cyclizations of Homo(polyprenyl)arene
analogs 31994 Biomimetic total Synthesis of terpenes and Steroids through
Polyene Cyclization 31995 Conclusion 328
References 328
SECTION III SHIKIMIC ACID BIOSYNTHETIC PATHWAY 331
10 Lignans Lignins and Resveratrols 333Yu Peng
101 Biosynthesis 3331011 Primary Metabolism of Shikimic acid and aromatic
amino acids 3331012 Lignans and Lignin 335
102 auxiliary‐assisted C(sp3)ndashH arylation Reactions in organic Synthesis 336
103 FriedelndashCrafts Reactions in organic Synthesis 344104 total Synthesis of Lignans by C(sp3)H arylation Reactions 353105 total Synthesis of Lignans and Polymeric Resveratrol by
FriedelndashCrafts Reactions 357106 Conclusion 375References 375
CoNtENtS xi
SECTION IV MIXED BIOSYNTHETIC PATHWAYSndash THE STORY OF ALKALOIDS 381
11 Ornithine and Lysine Alkaloids 383Sebastian Brauch Wouter S Veldmate and Floris P J T Rutjes
111 Biosynthesis of l‐ornithine and l‐Lysine alkaloids 3831111 Biosynthetic Formation of alkaloids
Derived from l‐ornithine 3831112 Biosynthetic Formation of alkaloids
Derived from l‐Lysine 388112 the asymmetric Mannich Reaction in organic Synthesis 392
1121 Chiral amines as Catalysts in asymmetric Mannich Reactions 3941122 Chiral Broslashnsted Bases as Catalysts in asymmetric
Mannich Reactions 3981123 Chiral Broslashnsted acids as Catalysts in asymmetric
Mannich Reactions 4041124 organometallic Catalysts in asymmetric Mannich Reactions 4081125 Biocatalytic asymmetric Mannich Reactions 413
113 Mannich and Related Reactions in the total Synthesis of l‐Lysine‐ and l‐ornithine‐Derived alkaloids 414
114 Conclusion 426References 427
12 Tyrosine Alkaloids 431Uwe Rinner and Mario Waser
121 Introduction 431122 Biosynthesis of tyrosine‐Derived alkaloids 431
1221 Phenylethylamines 4311222 Simple tetrahydroisoquinoline alkaloids 4331223 Modified Benzyltetrahydroisoquinoline alkaloids 4331224 Phenethylisoquinoline alkaloids 4361225 amaryllidaceae alkaloids 4381226 Biosynthetic overview of tyrosine‐Derived alkaloids 442
123 arylndasharyl Coupling Reactions 4421231 Copper‐Mediated arylndasharyl Bond Forming Reactions 4431232 Nickel‐Mediated arylndasharyl Bond Forming Reactions 4461233 Palladium‐Mediated arylndasharyl Bond Forming Reactions 4471234 transition Metal‐Catalyzed Couplings of Nonactivated
aryl Compounds 450124 Synthesis of tyrosine‐Derived alkaloids 456
1241 Synthesis of Modified Benzyltetrahydroisoquinoline alkaloids 4561242 Synthesis of Phenethylisoquinoline alkaloids 4601243 Synthesis of amaryllidaceae alkaloids 462
125 Conclusion 468References 469
13 Histidine and Histidine‐Like Alkaloids 473Ian S Young
131 Introduction 473132 Biosynthesis 473133 atom Economy and Protecting‐Group‐Free Chemistry 480
xii CoNtENtS
134 Challenging the Boundaries of Synthesis PIas 488135 Conclusion 497References 499
14 Anthranilic AcidndashTryptophan Alkaloids 502Zhen‐Yu Tang
141 Biosynthesis 502142 Divergent SynthesisndashCollective total Synthesis 508143 Collective total Synthesis of tryptophan‐Derived alkaloids 510
1431 Monoterpene Indole alkaloids 5101432 Bisindole alkaloids 512
References 517
15 Future Directions of Modern Organic Synthesis 519Jakob Pletz and Rolf Breinbauer
151 Introduction 519152 Enzymes in organic Synthesis Merging total
Synthesis with Biosynthesis 520153 Engineered Biosynthesis 526154 Diversity‐oriented Synthesis Biology‐oriented Synthesis
and Diverted total Synthesis 5331541 Diversity‐oriented Synthesis 5351542 Biology‐oriented Synthesis 5361543 Diverted total Synthesis 539
155 Conclusion 541References 545
INDEX 548
Elissavet E Anagnostaki Department of Chemistry Laboratory of Organic Chemistry Aristotle University of Thessaloniki Thessaloniki Greece and Research and Development Department Pharmathen SA Thessaloniki Greece
Stellios Arseniyadis School of Biological and Chemical Sciences Queen Mary University of London London United Kingdom
Louis Barriault Department of Chemistry University of Ottawa Ottawa Ontario Canada
Erica Benedetti Laboratoire de Chimie et Biochimie et Pharmacologiques et Toxicologiques CNRS-Universiteacute Paris Descartes Faculteacute des Sciences Fondamentales et Biomeacutedicales Paris France
Sebastian Brauch Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
Rolf Breinbauer Institute of Organic Chemistry Technische Universitaumlt Graz Graz Austria
Cyril Bressy Aix Marseille Universiteacute Centrale Marseille CNRS Marseille France
Martin Cordes Institute for Organic Chemistry and Center of Biomolecular Drug Research (BMWZ) Leibniz Universitaumlt Hannover Hannover Germany and Helmholtz Center for Infection Research (HZI) Hannover Germany
Paul E Floreancig Department of Chemistry Chevron Science Center University of Pittsburgh Pittsburgh PA USA
Tobias A M Gulder Department of Chemistry and Center for Integrated Protein Science Munich (CIPSM) Biosystems Chemistry Technische Universitaumlt Muumlnchen Munich Germany
Trond Vidar Hansen School of Pharmacy University of Oslo Oslo Norway
Kazuaki Ishihara Department of Biotechnology Graduate School of Engineering Nagoya University Nagoya Japan
Markus Kalesse Institute for Organic Chemistry and Center of Biomolecular Drug Research (BMWZ) Leibniz Universitaumlt Hannover Hannover Germany and Helmholtz Center for Infection Research (HZI) Hannover Germany
Xu‐Wen Li Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
Bastien Nay Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France
Yu Peng State Key Laboratory of Applied Organic Chemistry Lanzhou University Lanzhou China
Jakob Pletz Institute of Organic Chemistry Technische Universitaumlt Graz Graz Austria
Uwe Rinner Institute of Organic Chemistry Johannes Kepler University Linz Linz Austria and Department of Chemistry College of Science Sultan Qaboos University Muscat Oman
Floris P J T Rutjes Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
LIST oF CoNTRIBUToRS
xiv LIST OF CONTRIBUTORS
Franccediloise Schaefers Department of Chemistry and Center for Integrated Protein Science Munich (CIPSM) Biosystems Chemistry Technische Universitaumlt Muumlnchen Munich Germany
Michael Smietana Institut des Biomoleacutecules Max Mousseron CNRS Universiteacute de Montpellier ENSCM France
Zhen‐Yu Tang Department of Pharmaceutical Engineering College of Chemistry and Chemical Engineering Central South University Changsha China
Wouter S Veldmate Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
Anders Vik School of Pharmacy University of Oslo Oslo Norway
Mario Waser Institute of Organic Chemistry Johannes Kepler University Linz Linz Austria
Youwei Xie Max‐Planck‐Institut fuumlr Kohlenforschung Muumllheim Germany
Ian S Young Bristol‐Myers Squibb Company Chemical Development New Brunswick NJ USA
Alexandros L Zografos Department of Chemistry Laboratory of Organic Chemistry Aristotle University of Thessaloniki Thessaloniki Greece
There is pleasure in the pathless woodsthere is rapture in the lonely shore
there is society where none intrudesby the deep sea and music in its roar
I love not Man the less but Nature moreLord Byron
Preface
The first time I came across with the idea of editing a book that merges selected chapters of biosynthesis and total synthesis was when I was teaching postgraduate courses of natural product synthesis at Aristotle University of Thessaloniki This period I realized that the best way to teach youngsters synthesis was to start from the very origin of inspiration nature and its tools biosynthesis
Over the last decades biosynthesis is filling our gaps of understanding the complex mechanisms of nature and pro-vides useful sources of inspiration not only in the way natural products can be synthesized but also by directing synthetic chemists in developing atom‐economical efficient synthetic methods Several are the examples that mimic biosynthetic guidelines from modern iterative alkylations and aldol reactions to CH oxidations that compile nowadays the modern toolbox of organic synthesis
The handed book is constructed in the logic of presenting the parallel development of biosynthesis and organic meth-odology and how these can be applied in efficient syntheses of natural products The book is divided into four sections each representing the four major biosynthetic pathways of natural products namely acetate mevalonate shikimate biosynthetic pathways and the mixed biosynthetic pathways
of alkaloids These sections are divided into chapters that represent selected classes of natural products for example lipids sesquiterpenoids lignans etc Each of these chapters is further divided into three distinct subchapters (a) biosyn-thesis (b) methodological section and (c) application of the described methodology in the total synthesis of the described family of natural products By this way the readers can be focused in the direct comparison between biosyn-theses and the developed methodologies to construct the crucial for each class of natural product carbon bonds Although the book as it develops is focused on presenting the power of biosynthesis and how this power can be applied in providing inspiration for the efficient synthesis of natural products it was not the authors will to present only biomi-metic total syntheses but rather to exploit the modern synthetic methodologies and recognize their disabilities for further improvement
Of course this book will not have been realized without the excellent work of renowned scientists worldwide working either in the field of biosynthesis or total synthesis who collected the existing knowledge on biosynthesis analyzed the existing modern methodologies and presented a bouquet of selected total syntheses Throughout our
xvi PrEfACE
endeavor to complete this book I learned many things from their expertise but I also realized that only with tight collab-orations you can build long‐lasting friendships I would like to thank them all once again for their trust and effort to complete this book We all hope that the current work will contribute to a better understanding of the current status of
organic chemistry and to the discovery of novel strategies and tactics for the synthesis of natural products
Alexandros L ZografosSeptember 2015
Thessaloniki Greece
From Biosynthesis to Total Synthesis Strategies and Tactics for Natural Products First Edition Edited by Alexandros L Zografoscopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
11 FROM PRIMARY TO SECONDARY METABOLISM THE KEY BUILDING BLOCKS
111 Definitions
The primary and secondary metabolisms are traditionally distinguished by their distribution and utility in the living organism network Primary metabolites include carbohyshydrates lipids nucleic acids and proteins (or their amino acid constituents) and are shared by all living organisms on Earth They are transformed by common pathways which are studied by biochemistry (Fig 11) Secondary metabo-lites are structurally diverse compounds usually produced by a limited number of organisms which synthesize them for a special purpose like defense or signaling through specific biosynthetic pathways They are studied by natural product chemistry This distinction is not always so obvious and some compounds can be studied in the context of both primary and secondary metabolisms This is especially true nowadays with the use of genetic and biomolecular tools which tend to make natural product sciences more and more integrative However an important point to remember is that the primary metabolism furnishes key building blocks to the secondary metabolism It would be difficult to describe in detail the full biosynthetic pathshyways in this section We tried to organize the discussion as a vade mecum synthetically gathering information from extremely useful sources which will be cited at the end of this chapter
112 Energy Supply and Carbon Storing at the Early Stage of Metabolisms
The sunlight is essential to life except in some part of the deep oceans It provides energy for plant photosynthesis that splits molecules of water into protons and electrons and releases O
2 (Scheme 11) A proton gradient inside the plant
chloroplasts then drags a transmembrane ATP synthase comshyplex that produces adenosine triphosphate (ATP) while elecshytrons released from water are transferred to the coenzyme reducer nicotinamide adenine dinucleotide phosphate hydride (NADPH) A major function of chloroplasts is to fix CO
2 as a combination to ribulose‐15‐bisphosphate (RuBP)
performed by RuBP carboxylase (rubisco) forming an instable ldquoC
6rdquo β‐ketoacid This is cleaved into two molecules
of 3‐phosphoglycerate (3‐PGA) which is then reduced into 3‐phosphoglyceraldehyde (3‐PGAL a ldquoC
3rdquo triose phosshy
phate) during the Calvin cycle This is one of the major metabolites in the biosynthesis of carbohydrates like glucose and a biochemical mean for storing and retaining carbon atoms in the living cells
113 Glucose as a Starting Material Toward Key Building Blocks of the Secondary Metabolism
Glucose‐6‐phosphate arises from the phosphorylation of glucose It is the starting material of glycolysis an important process of the primary metabolism which consists in eight enzymatic reactions leading to pyruvic acid (PA)
FROM BIOSYNTHESES TO TOTAL SYNTHESES AN INTRODUCTION
Bastien Nay1 and Xu‐Wen Li2
1
1 Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France2 Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
2 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
(Scheme 12) Important intermediates for the secondary metabolism are produced during glycolysis Glucose glucose‐6‐phosphate and fructose‐6‐phosphate can be converted to other hexoses and pentoses that can be oligoshymerized and enter in the composition of heterosides Additionally fructose‐6‐phosphate connects the pentose
phosphate pathway leading to erythrose‐4‐phosphate toward shikimic acid which is a key metabolite in the biosynthesis of aromatic amino acids (phenylalanine tyrosine or C
6C
3
units) and C6C
1 phenolic compounds The next important
intermediate in glycolysis is 3‐PGAL which can be redishyrected toward methylerythritol‐4‐phosphate (mEP) in the
Primary metabolism Secondary metabolism
The field ofbiochemistry
The field ofnatural product
chemistry
Essential toliving organisms
Essential to theproducer organisms
under particularconditions
Nucleic acids (DNARNA) carbohydrates
lipids amino acidspeptides and proteins
Alkaloids terpenespolyketides
polyphenols andtheir heterosidic form
Biological effects(defense signaling)
Biosyntheticpathways
Main compoundclasses
FIGURE 11 Primary versus secondary metabolisms
PS-I
PS-IIendash
hν H2O H+ and O2
ATP synthase
NADP reductase
H+ (gradient)
H+ADP + Pi ATP
NADPHH+NADP+
H+
Thylakoidcompartment
Chloroplaststroma
Thylakoidmembrane
(a) Light dependent process
(b) Light independent process
CO2
RuBP
Rubisco
Unstable β-ketoacid3-PGA 13-diPGA
ATP
ADP3-PGAL
NADPHH+
NADP+
(Calvin cycle)
trioses tetroses pentoses hexoses (eg glucose) heptoses
Chloroplaststroma
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
CH2OPO32ndash CH2OPO3
2ndash CH2OPO32ndash
CH2OPO32ndash
OOH
HO
HOO
HO
HO2CCO2H
HO
CO2PO32ndash
HO
CHOHO
endash
Cytosol
SCHEME 11 The photosynthetic machinery (PS‐I and PS‐II photosystems I and II)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 3
chloroplast mEP is a starting block in the biosynthesis of terpenes through C
5 isoprene units (isopentenyl diphosphate
(IPP) and dimethylallyl diphosphate (DmAPP)) especially those in C
10 C
20 and C
40 terpenes 3‐PGA is a precursor of
serine and other amino acids while phosphoenolpyruvate (PEP) the precursor of PA is also an intermediate toward the previously mentioned shikimic acid Lastly PA is not only a precursor of the fundamental ldquoC
2rdquo acetyl coenzyme A
(AcCoA) unit but also an intermediate toward aliphatic amino acids and mEP
AcCoA is the building block of fatty acids polyketides and mevalonic acid (mVA) a cytosolic precursor of the C
5 isoprene units for the biosynthesis of terpenes in the
C15
and C30
series (mind it is different from the mEP pathway in product and in cell location) Finally AcCoA enters the citric acid or Krebs cycle which leads to several precursors of amino acids These are oxaloacetic acid precursor of aspartic acid through transamination (thus toward lysine as a nitrogenated C
5N linear unit and methishy
onine as a methyl supplier) and 2‐oxoglutaric acid preshycursor of glutamic acid (and subsequent derivatives such as ornithine as a nitrogenated C
4N linear unit) All these
amino acids are key precursors in the biosynthesis of many alkaloids
114 Reactions Involved in the Construction of Secondary Metabolites
most reactions occurring in the living cells are performed by specialized enzymes which have been classified in an intershynational nomenclature defined by an enzyme commission (EC) number There are six classes of enzymes depending on the biochemical reaction they catalyze EC‐1 oxidoreducshytases (catalyzing oxidoreduction reactions) EC‐2 transfershyases (catalyzing the transfer of functional groups) EC‐3 hydrolases (catalyzing hydrolysis) EC‐4 lyases (breaking bonds through another process than hydrolysis or oxidation leading to a new double bond or a new cycle) EC‐5 isomershyases (catalyzing the isomerization of a molecule) and EC‐6 ligases (forming a covalent bond between two molecules) many subclasses of these enzymes have been described depending on the type of atoms and functional groups involved in the reaction and if any on the cofactor used in this reaction For example several cofactors can be used by dehydrogenases like NAD(P)NAD(P)H FADFADH
2 or
FmNFmNH2 For a description of this classification the
reader can refer to specialized Internet websites like ExplorEnz [1] What is important to realize is that most enzymes are substrate specific and have been selected during
CHOHO
CO2H
O
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
HOOH
HO
HOH2CO
OHC
CH2OPO32ndash
OHHO
CO2H
HO
OH
OH
TYR
PHE
CO2H
NH2
TRP
CH2OPO32ndash
HO
HOH2COH
MEP
C10 C20 and C40 terpenes
CO2HHO
SER
GLY CYS
CO2HOPO3
2ndash
CH2OH
OHHO2C
MVA
C15 and C30 terpenes
Polyketides
Krebscycle
(citric acid)
VAL ALA ILE LEU
CO2H
O
HO2C
CO2HO
CO2H
ASP
LYS
GLU
Glucose-6-phosphate
Fructose-6-phosphate
Erythrose-4-phosphate
3-PGAL
OP2O63ndash OP2O6
3ndash
IPP DMAPP
3-PGA PEP PA
O SCoAAcCoA
3x
Cytosol
Cytosol
Chloroplasts
Chloroplasts
Glycolysis
C2
C5
Shikimic acid Oxaloaceticacid
2-Oxoglutaricacid
MET
THR
PRO
ARGORN
Alkaloids Alkaloids Alkaloids Alkaloids
Aminoacids
Peptidesproteins
Phenolics
C1
C5N C4N
ldquoCH3rdquo
(Indole)C2NC6C3C6C2N C6C1
Pentosephosphatepathway
SCHEME 12 The building block chart involving glycolysis and the Krebs cycle
4 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
evolution to perform specific transformations making natural products with often and yet unknown functions
Secondary metabolites arise from specific biosynthetic pathways which use the previously defined building blocks The bunch of organic reactions involved in these biosynshytheses allows the construction of natural product frameshyworks which are finally diversified through ldquodecorationrdquo steps (Scheme 13) It is not the purpose of this introductive chapter to describe in detail all biosynthetic pathways and the reader can refer to excellent books and articles which have been published elsewhere [2 3]
The reactions involved in the construction of natural product skeletons will be described later for representative classes of compounds The identification of the building block footprint in the natural product skeleton will be emphasized as much as possible sometimes referring to biogenetic speculations [4] After the framework construction the decoration steps will involve as diverse reactions as aliphatic CH oxidations (eg involving a cytochrome P
450 oxygenase) occasionally triggering a
rearrangement heteroatom alkylations (eg methylation by S‐adenosylmethionine) or allylation (by DmAPP) esterifications heteroatom or C‐glycosylations (leading to heterosides) radical couplings (especially for phenols) alcohol oxidations or ketone reductions amineketone transaminations alkene dihydroxylations or epoxidations oxidative halogenations BaeyerndashVilliger oxidations and further oxygenation steps At the end of the biosynthesis such transformations may totally hide the primary building block origin of natural products
115 Secondary Metabolisms
1151 Polyketides Polyketides (or polyacetates) are issued from the oligomerization of C
2 acetate units performed
by polyketide synthases (PKS) and leading to (C2)
n linear
intermediates [5 6] If the (C2)
n intermediates arise from
successive Claisen reactions performed by ketosynthase
domains (KS in nonreducing PKS) a highly reactive poly‐ β‐ketoacyl intermediate H(CH
2CO)
nOH is formed
leading to phenolic and aromatic products through further intramolecular Claisen condensations Furthermore highly reducing PKSs are made of specialized enzymatic subunits working in line or iteratively to functionalize each C
2 linker
bond as CH(OH)CH2 (by ketoreductases (KR)) then as
HCCH (by dehydratases (DH)) and as CH2CH
2 (by enoyl
reductases (ER)) leading to a high degree of functionalization of the final product (Fig 12) By these iterative sequences highly reduced polyketides which can be either linear macrocyclized or polycyclized depending on the reactivity of the biosynthetic intermediates can be formed [7] With the same logic fatty acids are also biosynthesized by fatty acid synthases
moreover the PKS enzyme can be hybridized with nonshyribosomal peptide synthetase (NRPS) domains (see also ldquoNRPS metabolites and peptidesrdquo in the ldquoAlkaloidsrdquo secshytion) leading to the acylation of an amino acid by the (C
2)
n
acyl intermediate As previously the functionalization of the acyl chain depends on the PKS enzyme and the PKSNRPS products are also extremely diversified (eg hirsutellone B Fig 12) [8]
1152 Terpenes Terpenes are derived from the oligoshymerization of the C
5 isoprene units DmAPP and IPP Both
precursors are prompt to generate either an allylic cation (the diphosphate is a good leaving group) or a tertiary carbocation respectively which makes the IPP easy to react with DmAPP (Scheme 14) This reaction happens in the active site of a terpene synthase which activates the deparshyture of the diphosphate group from DmAPP thanks to Lewis acid activation (a metal like mg2+ mn2+ or Co2+ is present in the enzyme active site [9]) This leads to geranyl (C
10
monoterpene precursor) or farnesyl (C15
sesquiterpene precursor) diphosphate depending on the location of the enzyme (chloroplast for the mEP pathway or cytosol for the mVA pathway) Geranylgeranyl (C
20 diterpene) and
Constructionreactions
Decoration reaction(functionalization)Building
blocksNatural product
frameworksNatural products
HH
OH OAcH
HO O OH
OHO
OBz
P450
P450
P450
P450
P450 P450 Oxidativeauxiliaryenzymes
Taxadiene
OP2O63ndash
OP2O63ndash
3times
(a)
(b)
10-Deacetylbaccatin III
DMAPP
IPPand electrophilic
cyclizations
SCHEME 13 (a) From building blocks to natural products and (b) the example of 10‐deacetylbaccatin III
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 5
farnesylfarnesyl (C30
triterpene precursor) diphosphates can also be obtained by further additions of IPP leading to longer linear intermediates
The cyclization of linear precursors is achieved by speshycialized cyclases which generate a poorly functionalized natural product framework [10 11] Auxiliary enzymes such as oxygenases then increase the complexity and the diversity
of compounds by further functionalization (Scheme 13b) [12] A high degree of oxidation can be observed in comshypounds like thapsigargin paclitaxel or bilobalide (Fig 13) The biosynthesis of this last compound for example involves a high oxygenation pattern two Wagnerndashmeerwein rearrangements and several oxidative cleavages leading to the loss of five carbons The resulting natural products can
KS
S-ACP
O O
KR
S-ACP
OH O
S-MAT
O
YesNo
R
R
R
YesNoDH
S-ACP
OR
YesNoER
S-ACP
OR
YesNoTE
OH
OR
New cycle
New cycleor
release
New cycleor
release
New cycleor
release
Release
R in
crem
ente
d by
two
carb
ons
HO
O
S-ACP
O
Claisen condensation (ndashCO2)
reduction (NADPH)
dehydration (ndashH2O)
+
reduction (NADPH)
hydrolysis (H2O)
O O NH
OH
OH
H H
H H
Hirsutellone B(mixed PKSNRPS
product)
OO
O
OHOH
HO OH
Norsolorinic acid
O
O
O
O CHO
OH
HH
H
H
HH
OHHemibrevetoxin B
OOHO
OHO
HOHO OH
Erythronolide A
OH
OH
HO
Panaxytriol
From ahexanoylstartingblock
H
H
Fromtyrosine
H
1C lost fromdecarboxylation
FIGURE 12 Chemical logic of polyketide construction leading to variable functionalization of the elongated acyl chain and examples of resulting chemical diversity ACP acyl carrier protein DH dehydratase ER enoyl reductase KR ketoreductase KS ketosynthase mAT malonyl acyl transferase TE thioesterase
DMAPPOP2O6
3ndashOP2O6
3ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
Geranyl diphosphate (GPP)2 times IPP
H H
Geranylgeranyl diphosphate (GGPP)
Examplesverticillane
casbanetaxane
phorbol
Exampleslabdane
pimaranekauraneabietane
aphidicolanegibberellane
IPP
SCHEME 14 Early assembly of C5 units in terpene biosynthesis leading to diterpenes (C
20)
6 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
thus be extremely modified with structures whose biogenetic origin is far from being obvious at first sight and cannot be determined without further experiments such as isotopic labeling
1153 Flavonoids Resveratrols Gallic Acids and Further Polyphenolics We have previously discussed the polyketide origin of some phenolic compounds based on the (C
2)
n motif Other polyphenols like gallic acids are directly
derived by the aromatization of shikimic acid (C6C
1 building
block Scheme 12) [13] The C6C
3 building blocks are availshy
able from the conversion of phenylalanine and tyrosine into cinnamic and p‐coumaric acids respectively and then by further hydroxylation steps (Scheme 15) These can dimerize into lignans (eg podophyllotoxin) [14 15] through radical processes or converted to low molecular weight compounds like eugenol coumarins or vanillin [16] The coenzyme A thioesters of these C
6C
3 acids can be used as initiator units
by specialized ketosynthases for an elongation by two acetyl units leading to aromatic polyketides like styrylpyrones or diarylheptanoids (eg curcumin) [17] Important comshypounds from this metabolism are flavonoids (C
6C
3C
6) [18]
and stilbenoids (C6C
2C
6) (a decarboxylation occurs during
the aryl cyclization) [19] which are synthesized by chalcone synthase and stilbene synthase respectively Flavonoids (eg catechin) and stilbenes (eg resveratrol) are present in large amounts in fruits and vegetables and may exert their radical scavenging properties in vivo
1154 Alkaloids Alkaloids are nitrogen‐containing compounds The nitrogen(s) can be involved in an amine function conferring basicity to the natural product (like ldquoalkalirdquo) or in less or nonbasic functions such as an amide a nitrile an isonitrile or an ammonium salt (quaternary amines) For amines protonation often occurs at physiological pH and may condition their biological activity In many cases the nitrogen is biogenetically derived from an amino acid We will thus discuss alkaloids according to their amino acid origin
Alkaloids Derived from the Krebs Cycle (Lysine and Ornithine Derived) As shown previously (Scheme 12) the Krebs cycle is a crucial metabolic process which leads to α‐ketoacids (oxaloacetic and 2‐oxoglutaric acids) Their enzymatic transamination affords the two amino acidsmdashaspartic acid and glutamic acidmdashwhich are the direct biosynthetic precursors of amino acids lysine and ornithine respectively These in turn produce cadaverine a ldquoC
5Nrdquo unit
and putrescine a ldquoC4Nrdquo unit which are major components
for the biosynthesis of important alkaloids as will be discussed later (Scheme 16) Additionally ornithine is a precursor of arginine another important amino acid
ornithine‐derived alkaloids (incorporating the c4n
unit) Putrescine is derived from the decarboxylation of ornithine and is a precursor of linear polyamines like spermshyine After enzymatic methylation of one amine of putrescine
C10 C15
C15
C10
C10
C30
CO2H
Chrysanthemic acid
OH
Menthol
O
HO
H
HOGlc
MeO2CLoganin (iridoid)
H
H
H
HH
OO
OParthenolide
O
O
OHOHO
OAcHO
OnC3H7
O
O
O
nC7H15
Thapsigargin
O
O
OO
H
H
O
Cleavage
H
H
Artemisinin
C20 C40
O
O
OO
OO
OHOHH
Bilobalide(highly rearrangedand missing 5 C)
CleavageH
Highoxidativecleavages
HO OAcH
AcO O OH
OOOBz
O
Ph
BzNH
OHFrom PHE Paclitaxel
C25O
H
HO
OHCH
OH Ophiobolin A
H
HO
H
H
HHCholesterol
(missing 3 C WM)
H
H H H
H
H
Hopene
β-Carotene
C30 - 3
C15
C20 - 5WM
WM
FIGURE 13 Chemical diversity in the terpene series (Wm Wagnerndashmeerwein shifts lost carbons bold bonds are remnant of primary building blocks)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 7
in the presence of S‐adenosylmethionine transamination of the other affords γ‐(N‐methylamino)aldehyde [20] The resulting cyclic iminium is a key intermediate in the formation of many medicinally important alkaloids such as
the plant‐derived compounds cocaine atropine or the calysshytegines [21 22] Indeed this iminium is a mannich acceptor which can react with various nucleophiles the first of those being the carbanion of acetyl‐CoA Thus after a stepwise
CO2H
RCinnamic acid (R=H)p-Coumaric acid (R=OH)
RO
PHETYR
OH
[H] [O]
lignans
OH
O
O
O
O
OMeMeO
OMe
C mdash C andor C mdash Oradical couplings
After E Zisomerization
for example Podophyllotoxin
O ORO
Coumarins(eg scopoletin)
[O]
Prenylationcyclizationcleavage[O]
O OO
Furocoumarins(eg psoralen)
RO
nAcetyl-CoA
thencyclization
n = 2 styrylpyrones
O O
OMe
n = 3 avonoids(from chalcone synthase)
C6C3
H
H H
From AcCoA
O
n = 3 stilbenoids(from stilbene synthase)
HO
OH
OHFrom AcCoA From AcCoA(ndashCO2)
Resveratrol
HO
OH
OH
OH
OH
H
Catechin
MeO Yangonin
From AcCoA
SCHEME 15 The phenylpropanoid biosynthetic pathways
LYS
CO2
Cadaverine
O NH
H2O
NH
Piperideineiminium
Pelletierine
AcAcCoA
CO2
NH
O
N
O
Pseudopelletierine
NH
Ph
OH
Ph
O
Lobeline
NH
δ+
δndash
N
H
H
N
H
OH
Lupinine
N
H N
H
(+)-Sparteine
N
NH
O (ndash)-Cytisine
NH
CO2H
Pipecolic acid
N
OHHO
OH
HO
H
Castanos permine
ORN
CO2
NH2 NH2 NH2 NH2
NH2
NH2
NH2
PutrescineO
NHMe
N-Methylpyrrolinium
2 timesAcCoA
NMe
ARG
n = 1 spermidinen = 2 homospermidine
NMe
O
HygrineCO2
CO2
MeN
O
[O]
TropinoneCO2
HNHO
OHOH
OH
Calystegine B2
MeN
OAtropine
O
Ph
HO
NMe
NMe
NMe
O
Cuscohygrine
HN
HN
NH2
( )n
O
n = 2
N
HHO
Retronecine
HH
H
H
H
HO
H H
H
SCHEME 16 Lysine‐ and ornithine‐derived alkaloid biosynthetic pathways (mind the structural similarities)
8 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
elongation by two AcCoA units either decarboxylation can occur leading to the acetonylpyrrolidine hygrine or a secshyond mannich reaction by the intramolecular attack of the acetoacetate anion onto an oxidation‐derived pyrrolinium leading to the tropane skeleton (tropinone) The acetoacetyl‐CoA intermediate can also react intermolecularly with another pyrrolinium cation leading to cuscohygrine after decarboxylation Finally the pyrrolizidine alkaloids [23] are derived from homospermidine which when submitted to terminal oxidative deamination leads to the bicyclic skelshyeton of retronecine and further Senecio alkaloids We can mention herein that ornithine is a biosynthetic precursor of arginine bearing a guanidine function which is an intermediate toward the toxic compounds tetrodotoxin and saxitoxin (not shown)
lysine‐derived alkaloids (incorporating the c5n
unit) From lysine to piperidine alkaloids the biosynthetic steps parallel the one previously described from ornithine Indeed the oxidative deamination of cadaverine affords a δ‐amino aldehyde which cyclizes through imine formation into piperideine Protonation results in a mannich acceptor which is able to react with various nucleophiles such as β‐ketothioester anions The first product of these reactions is pelletierine which can further react through an intramolecular mannich
reaction leading to pseudopelletierine Quinolizidines [24] can also be formed first from the mannich reaction of the piperideine acceptor with the corresponding enamine nucleoshyphile and then after additional transformation steps leading for example to lupinine sparteine or cytisine
Indolizidine alkaloids [15] such as castanospermine and swainsonine are formed from pipecolic acid an amino acid derived from lysine which can be elongated by malonyl‐CoA followed by ring closure When protonated these alkaloids are oxonium mimics strongly inhibiting glycosidases
Tyrosine‐ and Phenylalanine‐Derived Alkaloids Tyrosine and phenylalanine amino acids are bearing the phenylshyethylamine moiety of many medicinally relevant alkaloids Further hydroxylations on the aromatic carbocycle or on the aliphatic part can be observed methylations can occur on phenolic oxygens and on the amine leading to catecholamines (adrenaline noradrenaline dopamine) Arylethylamines are also usual to react with endogenous aldehydes through PictetndashSpengler reactions [25] leading to important biosynthetic intermediates (Scheme 17) like
bull Reticuline from the reaction with 4‐hydroxyphenylacetshyaldehyde toward benzyltetrahydroisoquinoline alkaloids
NH2
Phenylethylamines
RONH TetrahydroisoquinolinesRO
RʹCHO
Rʹ
NH
(CH2)n
MeO
HO
Benzyltetrahydroisoquinoline(n = 1) for example reticuline
orPhenethyltetrahydroisoquinoline
(n = 2) eg autumnaline
IntermolecularCmdashO phenol
couplings
Curarealkaloids
IntramolecularC mdash C phenol
couplings
ONMe
HHO
H
HO
Morphine
NMe
MeO
HO
MeOOH
Isoboldine
O
O
OMe
NO2
CO2H
Aristolochic acid
OMe
O
NHAcMeO
MeOMeO
Colchicine
N
O
O
OMe
OMeBerberine
IntramolecularCmdash C coupling
and furtheroxidative events
Rʹ derivedfrom PHE
Pictet-Spenglerreaction
TYR
HO
HO CHO
HNHO
HO
OH
OMeO
OH
NMe
[O]
GalantamineNorbelladine
Rʹ derived fromsecologanin
NAc
HO
HO
O
CO2MeH
H
H
OHIpecosideaglycone
Ipecac alkaloids
1ClostCmdash C cleavage
and ring extension
H
ROH
1C lost
Cleavage
SCHEME 17 Tyrosine‐derived alkaloid biosynthetic pathways (double head arrows figure bond cleavages during biosynthetic processes)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 9
morphine berberine tubocurarine isoboldine or the highly modified aristolochic acid [26 27]
bull Automnaline from the reaction with 3‐(4‐hydroxyphenyl)propanal toward phenylethyltetrahydroisoquinoline alkaloids colchicine cephalotaxine or schelhammerishycine [28 29]
bull Ipecoside from the reaction of dopamine with secoloshyganin toward terpene tetrahydroisoquinoline alkaloids ipecoside or emetine
Lastly norbelladine (top of Scheme 17) is issued from the reductive amination of 34‐dihydroxybenzaldehyde (derived from phenylalanine) with tyramine (derived from tyrosine) and constitutes a biosynthetic node leading to Amaryllidaceae alkaloids such as galantamine crinine or lycorine depending on the topology of phenolic couplings In all these biosynthetic routes radical phenolic couplings are key reactions for CC and CO bond formations and rearrangements [30 31]
Tryptophan‐Derived Indole and Indole Monoterpene Alkaloids As for alkaloids derived from tyrosine and phenylalanine those derived from tryptophan are formed after decarboxylation of the amino acid (into tryptamine) and possible hydroxylation of the aromatic carbocycle (eg serotonin) and N‐methylation (eg psilocin) As previously tryptamine can also react through PictetndashSpengler reactions to form tetrahydro‐β‐carbolines which can be aromatized for example into harmine (Scheme 18) [16]
When the aldehyde partner of the PictetndashSpengler reacshytion with tryptamine is the terpene secologanin strictosidine is formed as an entry toward the vast monoterpene indole alkaloids [32 33] Hydrolysis of the glucosidic part releases the strictosidine aglycone bearing an aldehyde while iminshyium formation and further cyclization and reduction can lead to ajmalicine (from oxocyclization) or yohimbine (from carshybocyclization) These alkaloids are referred to as from the Corynanthe type with the monoterpene carbon skeleton unmodified Although it misses one carbon and has a very
RO
Carbonylcompounds
TRPNH
NH2
Indolethylamines(eg serotonine)
RONH
NH
Rʹ NH
N
MeOPictet-Spenglerreaction for example Harmineβ-Carbolines
NH
NHH
O
MeO2C
MeO2C
OH
H
H
Rʹ derived from secologanin
Strictosidine aglycone
NH
N
OH
H
H
Ajmalicine
N
N
O
O
H
H
H
Strychnine (C lost)Corynanthe type
Aspidosperma type Iboga type
N
N
OH
OMe
Quinine (C lost)
N
NH CO2Me
Catharanthine
NH
N
CO2MeH
OAc
OH
Vindoline
Complextransformations
NH
NMe
HN
MeN
H
H
Chimonanthine
Cyclizationdimerization
Prenylationcyclization
NH
NMeHO2C
H
Lysergic acid
H
H
Ergot alkaloidsPyrroloindoles Indole monoterpene
1C lost
1C lostFrom AcCoA
SCHEME 18 Tryptophan‐derived alkaloid biosynthetic pathways (gray parts monoterpenic units)
10 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
different structure strychnine is related to the Corynanthe alkaloids incorporating two carbons from acetyl‐CoA Highly modified monoterpene skeletons are derived from the Corynanthe core through CC bond breaking and reorshyganization leading to Iboga‐type (eg catharanthine) and Aspidosperma‐type (eg vindoline) alkaloids The antishycancer drug vinblastine is a heterodimer resulting from the nucleophilic attack of vindoline on a mannich acceptor resulting from catharanthine found in madagascar perishywinkle (Catharanthus roseus) The heteroaromatic comshypounds ellipticine camptothecin and quinine are also derived from a Corynanthe‐type precursor although in this case the biosynthetic relationship may not be obvious due to deep modifications of the skeleton
Finally two important classes of compounds have to be mentioned since they have inspired many synthetic chemists The pyrroloindole alkaloids result from the cyclization of tryptamine as found in physostigmine (formed by a cationic mechanism after methylation in position 3 of the indole not shown) or in chimonanthine (presumably formed by a radical coupling mechanism Scheme 18) The ergot alkaloids are derived from the 33‐dimethylallylation on position 4 of the indole in trypshytophan whose further cyclization and oxidation processes afford the natural products (eg lysergic acid Scheme 18 and ergotamine) which have had important medical applications [34]
NRPS Metabolites and Peptides NRPS enzymes assemble amino acids including nonproteinogenic ones into oligopeptides The enzymes contain several modules and especially an adenylation domain (A) which specifically selects and activates the amino acid to be transferred as a thioester on the nearby peptidyl carrier protein (PCP) [2] A condensation module (C) then catalyzes the formation of the peptide bonds between the newly introduced amino acyl‐PCP (bearing a free amine) and the elongated peptidyl‐PCP thioester At the end of the elongation a cyclization can occur into cyclopeptides but the peptide can also be
transferred to auxiliary enzymes like methyltransferases glycosyltransferases or oxidases (vancomycins are typical products of such functionalizations) [35 36] The formation of heterocycles is also frequently encountered in this metabolism as in penicillins that are derived from the tripeptide α‐aminoadipoyl‐cysteinyl‐valine or telomestatin (Fig 14) [2]
Other Alkaloid Origins There are many other nitrogen sources involved in alkaloid biosyntheses for example nicotinic acid (originated from aspartic acid and intershymediate in nicotine and anabasine biosyntheses) and anthranilic acid (originated from tryptophan and intermedishyate toward acridines or aurachins) The amination reaction (eg through transamination of carbonyl compounds) is also a way to introduce nitrogens in natural products for example from fatty acids steroids (toward Solanum alkaloids or cyclopamines) or other terpenoids (aconitine and atisine have diterpene skeletons while Daphniphyllum alkaloids are triterpene derivatives) Finally nucleic acids can also be precursors of alkaloids like the well‐known caffeine
12 FROM BIOSYNTHESIS TO TOTAL SYNTHESIS STRATEGIES TOWARD THE NATURAL PRODUCT CHEMICAL SPACE
121 The Chemical Space of Natural Products
Natural products occupy an important place in human com munities as demonstrated by their vast use from ancient times to nowadays like dyes fibers oils pershyfumes agrochemicals or drugs Broadly both primary and secondary metabolites could be classified as natural products while the latter as discussed previously are usushyally regarded as the ldquonatural productsrdquo owing to their comshyplexity and diversity arising from a variety of biosynthetic pathways The structural chemical diversity found among
N
S
CO2HO
HNPhH2C
O
Penicillin G
OH
HN
HN
O
O
ONH2
O
OHO
NHMe
OCl
O
NH
NH
NH
O
ClHO
O
HN
H
H
HOOH
OHHO2C
Vancomycin aglycone
ON
NO
O
NN
O
O
N
N O
Me
S
N N
OMeH
Telomestatin
L-AAA-L-CYS-D-VAL
Enzymes
FIGURE 14 Structural diversity of nonribosomal peptide compounds (AAA α‐aminoadipic acid)
FROm BIOSyNTHESIS TO TOTAL SyNTHESIS STRATEGIES TOWARD THE NATuRAL PRODuCT CHEmICAL SPACE 11
all living organisms defining the chemical space of natural products [37] is the consequence of their evolution occurshyring as an adaptation of organisms to their environment It is commonly believed that secondary metabolites are proshyduced as messengers by living organisms or as weapons against enemies and thus they should have certain biological activities in a medicinal point of view [38] Indeed natural products are regarded as one of the main sources of medicines (Fig 15) From the traditional medicinal extracts to every single bioactive molecule the methods of extraction purification identification and biological investigation of natural products have been well established Their complex structures and interesting properties have attracted synthetic chemists to accomplish their total syntheses and that of medicinally relevant anashylogs sometimes in the industrial context [39] Thus tarshygeting the chemical space of natural products has never been more relevant than today Although the discovery of natural products demands time and labor‐consuming manipulations it is worth to notice that the knowledge on this chemical space is still continually growing while biological advances allow for discovering and undershystanding potential targets However increasing the chemical space of human‐made compounds based on natural products should benefit from transdisciplinary colshylaborations such as the use of coupled biosynthetic and chemical synthetic methods to design original ldquounnatural natural productsrdquo [40]
122 The Biosynthetic Pathways as an Inspiration for Synthetic Challenges
1221 Precursor‐Directed Biosyntheses and Mutashysynthetic Strategies to Increase the Chemical Space of Natural Products As the genetics and biochemistry of natural product biosynthesis are better understood novel biosynthetic techniques have been developed to study and generate new diversity in natural product analogs (Scheme 19) Precursor‐directed biosynthesis (PDB) is considered as the earliest example of combining chemical and biological methods for the generation of complex natural product analogs [41 42] This approach compared with the biosynthetic pathway of wild‐type metabolites (Scheme 19a) involves the feeding of analogs of the natural biosynthetic building blocks to the living organisms (Scheme 19b) usually bacteria or fungi which incorporate the modified precursors into the biosynthesized compound mutasynthesis also termed as mutational biosynthesis (mBS) involves the inactivation of a key step of the bioshysynthesis in a mutant microorganism (Scheme 19c) which can then be fed by various modified or advanced building blocks (mutasynthons Scheme 19d) [43] These mutasshyynthons could not be incorporated by the wild type due to specificities of the enzymatic machinery Build up on PDB mBS eliminates the natural biosynthetic intermediate thus generating a less complex mixture of metabolites and making the purification or yield of target products better
O NH
OHO
O
OO
O
O
OH
HOO
O O
OH
HOO
NH ONH2
O
ONH
NH
O
OCl Cl
O
O
OH
HN
HN
HN
HN
OO
HO
HN
OH
OHOH
O
O
HO
HO
OHO
O
OHH2N
O
OH3C
H
O
H
CH3
CH3
H
O O
NO
H
O
HO
O
NO
O
NH
HN
OHO
HONH
ON
H
HO
O
H2N
O OH
ONH
OH
ON OHO
OH
HO OS OH
OOOH
OHO
O
ON
OO
O
HO
O
O OH
OO
Micafunginantifungal
Rapamycinimmunosuppressive
Galantamineanti-Alzheimer
Taxolanticancer
Artemisininantimalarial
Vancomycinantibiotic
FIGURE 15 Some famous natural products currently used as drugs
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product
Copyright copy 2016 by John Wiley amp Sons Inc All rights reserved
Published by John Wiley amp Sons Inc Hoboken New JerseyPublished simultaneously in Canada
No part of this publication may be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechanical photocopying recording scanning or otherwise except as permitted under Section 107 or 108 of the 1976 United States Copyright Act without either the prior written permission of the Publisher or authorization through payment of the appropriate per‐copy fee to the Copyright Clearance Center Inc 222 Rosewood Drive Danvers MA 01923 (978) 750‐8400 fax (978) 750‐4470 or on the web at wwwcopyrightcom Requests to the Publisher for permission should be addressed to the Permissions Department John Wiley amp Sons Inc 111 River Street Hoboken NJ 07030 (201) 748‐6011 fax (201) 748‐6008 or online at httpwwwwileycomgopermissions
Limit of LiabilityDisclaimer of Warranty While the publisher and author have used their best efforts in preparing this book they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose No warranty may be created or extended by sales representatives or written sales materials The advice and strategies contained herein may not be suitable for your situation You should consult with a professional where appropriate Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages including but not limited to special incidental consequential or other damages
For general information on our other products and services or for technical support please contact our Customer Care Department within the United States at (800) 762‐2974 outside the United States at (317) 572‐3993 or fax (317) 572‐4002
Wiley also publishes its books in a variety of electronic formats Some content that appears in print may not be available in electronic formats For more information about Wiley products visit our web site at wwwwileycom
Library of Congress Cataloging‐in‐Publication Data
Names Zografos Alexandros L editorTitle From biosynthesis to total synthesis strategies and tactics for natural products edited by Alexandros L ZografosDescription Hoboken New Jersey John Wiley amp Sons Inc [2016] | Includes bibliographical references and indexIdentifiers LCCN 2015037375 (print) | LCCN 2015047240 (ebook) | ISBN 9781118751732 (cloth) | ISBN 9781118753569 (Adobe PDF) | ISBN 9781118753637 (ePub)Subjects LCSH Organic compoundsndashSynthesis | BiosynthesisClassification LCC QD262 F76 2016 (print) | LCC QD262 (ebook) | DDC 57245ndashdc23LC record available at httplccnlocgov2015037375
Set in 1012pt Times by SPi Global Pondicherry India
Printed in the United States of America
10 9 8 7 6 5 4 3 2 1
Dedicated to my mother father and wife
LIST OF CONTRIBUTORS xiii
PREFACE xv
1 From Biosyntheses to Total Syntheses An Introduction 1Bastien Nay and Xu‐Wen Li
11 From Primary to Secondary Metabolism the Key Building Blocks 1111 Definitions 1112 Energy Supply and Carbon Storing at the Early Stage
of Metabolisms 1113 Glucose as a Starting Material toward Key Building Blocks
of the Secondary Metabolism 1114 Reactions Involved in the Construction of Secondary Metabolites 3115 Secondary Metabolisms 4
12 From Biosynthesis to total Synthesis Strategies toward the Natural Product Chemical Space 10121 the Chemical Space of Natural Products 10122 the Biosynthetic Pathways as an Inspiration
for Synthetic Challenges 11123 the Science of total Synthesis 14124 Conclusion a Journey in the Future of total Synthesis 16
References 16
SECTION I ACETATE BIOSYNTHETIC PATHWAY 19
2 Polyketides 21Franccediloise Schaefers Tobias A M Gulder Cyril Bressy Michael Smietana Erica Benedetti Stellios Arseniyadis Markus Kalesse and Martin Cordes
21 Polyketide Biosynthesis 21211 Introduction 21212 assembly of acetateMalonate‐Derived Metabolites 23213 Classification of Polyketide Biosynthetic Machineries 23214 Conclusion 39
References 40
CONTENTS
viii CoNtENtS
22 Synthesis of Polyketides 44221 asymmetric alkylation Reactions 44222 applications of asymmetric alkylation Reactions in total Synthesis
of Polyketides and Macrolides 60References 8323 Synthesis of Polyketides‐Focus on Macrolides 87
231 Introduction 87232 Stereoselective Synthesis of 13‐Diols asymmetric aldol Reactions 88233 Stereoselective Synthesis of 13‐Diols asymmetric Reductions 106234 application of Stereoselective Synthesis of 13‐Diols in
the total Synthesis of Macrolides 117235 Conclusion 126
References 126
3 Fatty Acids and Their Derivatives 130Anders Vik and Trond Vidar Hansen
31 Introduction 13032 Biosynthesis 130
321 Fatty acids and Lipids 130322 Polyunsaturated Fatty acids 134323 Mediated oxidations of ω‐3 and ω‐6 Polyunsaturated
Fatty acids 13533 Synthesis of ω‐3 and ω‐6 all‐Z Polyunsaturated Fatty acids 140
331 Synthesis of Polyunsaturated Fatty acids by the Wittig Reaction or by the Polyyne Semihydrogenation 140
332 Synthesis of Polyunsaturated Fatty acids via Cross Coupling Reactions 143
34 applications in total Synthesis of Polyunsaturated Fatty acids 145341 Palladium‐Catalyzed Cross Coupling Reactions 145342 Biomimetic transformations of Polyunsaturated Fatty acids 149343 Landmark total Syntheses 153344 Synthesis of Leukotriene B
5 158
35 Conclusion 160acknowledgments 160References 160
4 Polyethers 162Youwei Xie and Paul E Floreancig
41 Introduction 16242 Biosynthesis 162
421 Ionophore antibiotics 162422 Marine Ladder toxins 165423 annonaceous acetogenins and terpene Polyethers 165
43 Epoxide Reactivity and Stereoselective Synthesis 166431 Regiocontrol in Epoxide‐opening Reactions 166432 Stereoselective Epoxide Synthesis 172
44 applications to total Synthesis 176441 acid‐Mediated transformations 176442 Cascades via Epoxonium Ion Formation 179443 Cyclizations under Basic Conditions 181444 Cyclization in Water 182
45 Conclusions 183References 184
CoNtENtS ix
SECTION II MEVALONATE BIOSYNTHETIC PATHWAY 187
5 From Acetate to Mevalonate and Deoxyxylulose Phosphate Biosynthetic Pathways An Introduction to Terpenoids 189Alexandros L Zografos and Elissavet E Anagnostaki
51 Introduction 18952 Mevalonic acid Pathway 19153 Mevalonate‐Independent Pathway 19254 Conclusion 194References 194
6 Monoterpenes and Iridoids 196Mario Waser and Uwe Rinner
61 Introduction 19662 Biosynthesis 196
621 acyclic Monoterpenes 197622 Cyclic Monoterpenes 197623 Iridoids 200624 Irregular Monoterpenes 202
63 asymmetric organocatalysis 203631 Introduction and Historical Background 204632 Enamine Iminium and Singly occupied Molecular
orbital activation 207633 Chiral (Broslashnsted) acids and H‐Bonding Donors 213634 Chiral BroslashnstedLewis Bases and Nucleophilic Catalysis 218635 asymmetric Phase‐transfer Catalysis 220
64 organocatalysis in the total Synthesis of Iridoids and Monoterpenoid Indole alkaloids 225641 (+)‐Geniposide and 7‐Deoxyloganin 226642 (ndash)‐Brasoside and (ndash)‐Littoralisone 227643 (+)‐Mitsugashiwalactone 229644 alstoscholarine 229645 (+)‐aspidospermidine and (+)‐Vincadifformine 230646 (+)‐Yohimbine 230
65 Conclusion 231References 231
7 Sesquiterpenes 236Alexandros L Zografos and Elissavet E Anagnostaki
71 Biosynthesis 23672 Cycloisomerization Reactions in organic Synthesis 244
721 Enyne Cycloisomerization 245722 Diene Cycloisomerization 257
73 application of Cycloisomerizations in the total Synthesis of Sesquiterpenoids 266731 Picrotoxane Sesquiterpenes 266732 aromadendrane Sesquiterpenes Epiglobulol 267733 CubebolndashCubebenes Sesquiterpenes 267734 Ventricos‐7(13)‐ene 270735 Englerins 271736 Echinopines 271737 Cyperolone 273
x CoNtENtS
738 Diverse Sesquiterpenoids 27674 Conclusion 276References 276
8 Diterpenes 279Louis Barriault
81 Introduction 27982 Biosynthesis of Diterpenes Based on Cationic Cyclizations
12‐Shifts and transannular Processes 27983 Pericyclic Reactions and their application in the Synthesis
of Selected Diterpenoids 284831 Dielsndashalder Reaction and Its application in the total
Synthesis of Diterpenes 284832 Cascade Pericyclic Reactions and their application in the total
Synthesis of Diterpenes 29184 Conclusion 293
References 294
9 Higher Terpenes and Steroids 296Kazuaki Ishihara
91 Introduction 29692 Biosynthesis 29693 Cascade Polyene Cyclizations 303
931 Diastereoselective Polyene Cyclizations 303932 ldquoChiral proton (H+)rdquo‐Induced Polyene Cyclizations 304933 ldquoChiral Metal Ionrdquo‐Induced Polyene Cyclizations 308934 ldquoChiral Halonium Ion (X+)rdquo‐Induced Polyene Cyclizations 313935 ldquoChiral Carbocationrdquo‐Induced Polyene Cyclizations 319936 Stereoselective Cyclizations of Homo(polyprenyl)arene
analogs 31994 Biomimetic total Synthesis of terpenes and Steroids through
Polyene Cyclization 31995 Conclusion 328
References 328
SECTION III SHIKIMIC ACID BIOSYNTHETIC PATHWAY 331
10 Lignans Lignins and Resveratrols 333Yu Peng
101 Biosynthesis 3331011 Primary Metabolism of Shikimic acid and aromatic
amino acids 3331012 Lignans and Lignin 335
102 auxiliary‐assisted C(sp3)ndashH arylation Reactions in organic Synthesis 336
103 FriedelndashCrafts Reactions in organic Synthesis 344104 total Synthesis of Lignans by C(sp3)H arylation Reactions 353105 total Synthesis of Lignans and Polymeric Resveratrol by
FriedelndashCrafts Reactions 357106 Conclusion 375References 375
CoNtENtS xi
SECTION IV MIXED BIOSYNTHETIC PATHWAYSndash THE STORY OF ALKALOIDS 381
11 Ornithine and Lysine Alkaloids 383Sebastian Brauch Wouter S Veldmate and Floris P J T Rutjes
111 Biosynthesis of l‐ornithine and l‐Lysine alkaloids 3831111 Biosynthetic Formation of alkaloids
Derived from l‐ornithine 3831112 Biosynthetic Formation of alkaloids
Derived from l‐Lysine 388112 the asymmetric Mannich Reaction in organic Synthesis 392
1121 Chiral amines as Catalysts in asymmetric Mannich Reactions 3941122 Chiral Broslashnsted Bases as Catalysts in asymmetric
Mannich Reactions 3981123 Chiral Broslashnsted acids as Catalysts in asymmetric
Mannich Reactions 4041124 organometallic Catalysts in asymmetric Mannich Reactions 4081125 Biocatalytic asymmetric Mannich Reactions 413
113 Mannich and Related Reactions in the total Synthesis of l‐Lysine‐ and l‐ornithine‐Derived alkaloids 414
114 Conclusion 426References 427
12 Tyrosine Alkaloids 431Uwe Rinner and Mario Waser
121 Introduction 431122 Biosynthesis of tyrosine‐Derived alkaloids 431
1221 Phenylethylamines 4311222 Simple tetrahydroisoquinoline alkaloids 4331223 Modified Benzyltetrahydroisoquinoline alkaloids 4331224 Phenethylisoquinoline alkaloids 4361225 amaryllidaceae alkaloids 4381226 Biosynthetic overview of tyrosine‐Derived alkaloids 442
123 arylndasharyl Coupling Reactions 4421231 Copper‐Mediated arylndasharyl Bond Forming Reactions 4431232 Nickel‐Mediated arylndasharyl Bond Forming Reactions 4461233 Palladium‐Mediated arylndasharyl Bond Forming Reactions 4471234 transition Metal‐Catalyzed Couplings of Nonactivated
aryl Compounds 450124 Synthesis of tyrosine‐Derived alkaloids 456
1241 Synthesis of Modified Benzyltetrahydroisoquinoline alkaloids 4561242 Synthesis of Phenethylisoquinoline alkaloids 4601243 Synthesis of amaryllidaceae alkaloids 462
125 Conclusion 468References 469
13 Histidine and Histidine‐Like Alkaloids 473Ian S Young
131 Introduction 473132 Biosynthesis 473133 atom Economy and Protecting‐Group‐Free Chemistry 480
xii CoNtENtS
134 Challenging the Boundaries of Synthesis PIas 488135 Conclusion 497References 499
14 Anthranilic AcidndashTryptophan Alkaloids 502Zhen‐Yu Tang
141 Biosynthesis 502142 Divergent SynthesisndashCollective total Synthesis 508143 Collective total Synthesis of tryptophan‐Derived alkaloids 510
1431 Monoterpene Indole alkaloids 5101432 Bisindole alkaloids 512
References 517
15 Future Directions of Modern Organic Synthesis 519Jakob Pletz and Rolf Breinbauer
151 Introduction 519152 Enzymes in organic Synthesis Merging total
Synthesis with Biosynthesis 520153 Engineered Biosynthesis 526154 Diversity‐oriented Synthesis Biology‐oriented Synthesis
and Diverted total Synthesis 5331541 Diversity‐oriented Synthesis 5351542 Biology‐oriented Synthesis 5361543 Diverted total Synthesis 539
155 Conclusion 541References 545
INDEX 548
Elissavet E Anagnostaki Department of Chemistry Laboratory of Organic Chemistry Aristotle University of Thessaloniki Thessaloniki Greece and Research and Development Department Pharmathen SA Thessaloniki Greece
Stellios Arseniyadis School of Biological and Chemical Sciences Queen Mary University of London London United Kingdom
Louis Barriault Department of Chemistry University of Ottawa Ottawa Ontario Canada
Erica Benedetti Laboratoire de Chimie et Biochimie et Pharmacologiques et Toxicologiques CNRS-Universiteacute Paris Descartes Faculteacute des Sciences Fondamentales et Biomeacutedicales Paris France
Sebastian Brauch Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
Rolf Breinbauer Institute of Organic Chemistry Technische Universitaumlt Graz Graz Austria
Cyril Bressy Aix Marseille Universiteacute Centrale Marseille CNRS Marseille France
Martin Cordes Institute for Organic Chemistry and Center of Biomolecular Drug Research (BMWZ) Leibniz Universitaumlt Hannover Hannover Germany and Helmholtz Center for Infection Research (HZI) Hannover Germany
Paul E Floreancig Department of Chemistry Chevron Science Center University of Pittsburgh Pittsburgh PA USA
Tobias A M Gulder Department of Chemistry and Center for Integrated Protein Science Munich (CIPSM) Biosystems Chemistry Technische Universitaumlt Muumlnchen Munich Germany
Trond Vidar Hansen School of Pharmacy University of Oslo Oslo Norway
Kazuaki Ishihara Department of Biotechnology Graduate School of Engineering Nagoya University Nagoya Japan
Markus Kalesse Institute for Organic Chemistry and Center of Biomolecular Drug Research (BMWZ) Leibniz Universitaumlt Hannover Hannover Germany and Helmholtz Center for Infection Research (HZI) Hannover Germany
Xu‐Wen Li Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
Bastien Nay Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France
Yu Peng State Key Laboratory of Applied Organic Chemistry Lanzhou University Lanzhou China
Jakob Pletz Institute of Organic Chemistry Technische Universitaumlt Graz Graz Austria
Uwe Rinner Institute of Organic Chemistry Johannes Kepler University Linz Linz Austria and Department of Chemistry College of Science Sultan Qaboos University Muscat Oman
Floris P J T Rutjes Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
LIST oF CoNTRIBUToRS
xiv LIST OF CONTRIBUTORS
Franccediloise Schaefers Department of Chemistry and Center for Integrated Protein Science Munich (CIPSM) Biosystems Chemistry Technische Universitaumlt Muumlnchen Munich Germany
Michael Smietana Institut des Biomoleacutecules Max Mousseron CNRS Universiteacute de Montpellier ENSCM France
Zhen‐Yu Tang Department of Pharmaceutical Engineering College of Chemistry and Chemical Engineering Central South University Changsha China
Wouter S Veldmate Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
Anders Vik School of Pharmacy University of Oslo Oslo Norway
Mario Waser Institute of Organic Chemistry Johannes Kepler University Linz Linz Austria
Youwei Xie Max‐Planck‐Institut fuumlr Kohlenforschung Muumllheim Germany
Ian S Young Bristol‐Myers Squibb Company Chemical Development New Brunswick NJ USA
Alexandros L Zografos Department of Chemistry Laboratory of Organic Chemistry Aristotle University of Thessaloniki Thessaloniki Greece
There is pleasure in the pathless woodsthere is rapture in the lonely shore
there is society where none intrudesby the deep sea and music in its roar
I love not Man the less but Nature moreLord Byron
Preface
The first time I came across with the idea of editing a book that merges selected chapters of biosynthesis and total synthesis was when I was teaching postgraduate courses of natural product synthesis at Aristotle University of Thessaloniki This period I realized that the best way to teach youngsters synthesis was to start from the very origin of inspiration nature and its tools biosynthesis
Over the last decades biosynthesis is filling our gaps of understanding the complex mechanisms of nature and pro-vides useful sources of inspiration not only in the way natural products can be synthesized but also by directing synthetic chemists in developing atom‐economical efficient synthetic methods Several are the examples that mimic biosynthetic guidelines from modern iterative alkylations and aldol reactions to CH oxidations that compile nowadays the modern toolbox of organic synthesis
The handed book is constructed in the logic of presenting the parallel development of biosynthesis and organic meth-odology and how these can be applied in efficient syntheses of natural products The book is divided into four sections each representing the four major biosynthetic pathways of natural products namely acetate mevalonate shikimate biosynthetic pathways and the mixed biosynthetic pathways
of alkaloids These sections are divided into chapters that represent selected classes of natural products for example lipids sesquiterpenoids lignans etc Each of these chapters is further divided into three distinct subchapters (a) biosyn-thesis (b) methodological section and (c) application of the described methodology in the total synthesis of the described family of natural products By this way the readers can be focused in the direct comparison between biosyn-theses and the developed methodologies to construct the crucial for each class of natural product carbon bonds Although the book as it develops is focused on presenting the power of biosynthesis and how this power can be applied in providing inspiration for the efficient synthesis of natural products it was not the authors will to present only biomi-metic total syntheses but rather to exploit the modern synthetic methodologies and recognize their disabilities for further improvement
Of course this book will not have been realized without the excellent work of renowned scientists worldwide working either in the field of biosynthesis or total synthesis who collected the existing knowledge on biosynthesis analyzed the existing modern methodologies and presented a bouquet of selected total syntheses Throughout our
xvi PrEfACE
endeavor to complete this book I learned many things from their expertise but I also realized that only with tight collab-orations you can build long‐lasting friendships I would like to thank them all once again for their trust and effort to complete this book We all hope that the current work will contribute to a better understanding of the current status of
organic chemistry and to the discovery of novel strategies and tactics for the synthesis of natural products
Alexandros L ZografosSeptember 2015
Thessaloniki Greece
From Biosynthesis to Total Synthesis Strategies and Tactics for Natural Products First Edition Edited by Alexandros L Zografoscopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
11 FROM PRIMARY TO SECONDARY METABOLISM THE KEY BUILDING BLOCKS
111 Definitions
The primary and secondary metabolisms are traditionally distinguished by their distribution and utility in the living organism network Primary metabolites include carbohyshydrates lipids nucleic acids and proteins (or their amino acid constituents) and are shared by all living organisms on Earth They are transformed by common pathways which are studied by biochemistry (Fig 11) Secondary metabo-lites are structurally diverse compounds usually produced by a limited number of organisms which synthesize them for a special purpose like defense or signaling through specific biosynthetic pathways They are studied by natural product chemistry This distinction is not always so obvious and some compounds can be studied in the context of both primary and secondary metabolisms This is especially true nowadays with the use of genetic and biomolecular tools which tend to make natural product sciences more and more integrative However an important point to remember is that the primary metabolism furnishes key building blocks to the secondary metabolism It would be difficult to describe in detail the full biosynthetic pathshyways in this section We tried to organize the discussion as a vade mecum synthetically gathering information from extremely useful sources which will be cited at the end of this chapter
112 Energy Supply and Carbon Storing at the Early Stage of Metabolisms
The sunlight is essential to life except in some part of the deep oceans It provides energy for plant photosynthesis that splits molecules of water into protons and electrons and releases O
2 (Scheme 11) A proton gradient inside the plant
chloroplasts then drags a transmembrane ATP synthase comshyplex that produces adenosine triphosphate (ATP) while elecshytrons released from water are transferred to the coenzyme reducer nicotinamide adenine dinucleotide phosphate hydride (NADPH) A major function of chloroplasts is to fix CO
2 as a combination to ribulose‐15‐bisphosphate (RuBP)
performed by RuBP carboxylase (rubisco) forming an instable ldquoC
6rdquo β‐ketoacid This is cleaved into two molecules
of 3‐phosphoglycerate (3‐PGA) which is then reduced into 3‐phosphoglyceraldehyde (3‐PGAL a ldquoC
3rdquo triose phosshy
phate) during the Calvin cycle This is one of the major metabolites in the biosynthesis of carbohydrates like glucose and a biochemical mean for storing and retaining carbon atoms in the living cells
113 Glucose as a Starting Material Toward Key Building Blocks of the Secondary Metabolism
Glucose‐6‐phosphate arises from the phosphorylation of glucose It is the starting material of glycolysis an important process of the primary metabolism which consists in eight enzymatic reactions leading to pyruvic acid (PA)
FROM BIOSYNTHESES TO TOTAL SYNTHESES AN INTRODUCTION
Bastien Nay1 and Xu‐Wen Li2
1
1 Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France2 Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
2 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
(Scheme 12) Important intermediates for the secondary metabolism are produced during glycolysis Glucose glucose‐6‐phosphate and fructose‐6‐phosphate can be converted to other hexoses and pentoses that can be oligoshymerized and enter in the composition of heterosides Additionally fructose‐6‐phosphate connects the pentose
phosphate pathway leading to erythrose‐4‐phosphate toward shikimic acid which is a key metabolite in the biosynthesis of aromatic amino acids (phenylalanine tyrosine or C
6C
3
units) and C6C
1 phenolic compounds The next important
intermediate in glycolysis is 3‐PGAL which can be redishyrected toward methylerythritol‐4‐phosphate (mEP) in the
Primary metabolism Secondary metabolism
The field ofbiochemistry
The field ofnatural product
chemistry
Essential toliving organisms
Essential to theproducer organisms
under particularconditions
Nucleic acids (DNARNA) carbohydrates
lipids amino acidspeptides and proteins
Alkaloids terpenespolyketides
polyphenols andtheir heterosidic form
Biological effects(defense signaling)
Biosyntheticpathways
Main compoundclasses
FIGURE 11 Primary versus secondary metabolisms
PS-I
PS-IIendash
hν H2O H+ and O2
ATP synthase
NADP reductase
H+ (gradient)
H+ADP + Pi ATP
NADPHH+NADP+
H+
Thylakoidcompartment
Chloroplaststroma
Thylakoidmembrane
(a) Light dependent process
(b) Light independent process
CO2
RuBP
Rubisco
Unstable β-ketoacid3-PGA 13-diPGA
ATP
ADP3-PGAL
NADPHH+
NADP+
(Calvin cycle)
trioses tetroses pentoses hexoses (eg glucose) heptoses
Chloroplaststroma
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
CH2OPO32ndash CH2OPO3
2ndash CH2OPO32ndash
CH2OPO32ndash
OOH
HO
HOO
HO
HO2CCO2H
HO
CO2PO32ndash
HO
CHOHO
endash
Cytosol
SCHEME 11 The photosynthetic machinery (PS‐I and PS‐II photosystems I and II)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 3
chloroplast mEP is a starting block in the biosynthesis of terpenes through C
5 isoprene units (isopentenyl diphosphate
(IPP) and dimethylallyl diphosphate (DmAPP)) especially those in C
10 C
20 and C
40 terpenes 3‐PGA is a precursor of
serine and other amino acids while phosphoenolpyruvate (PEP) the precursor of PA is also an intermediate toward the previously mentioned shikimic acid Lastly PA is not only a precursor of the fundamental ldquoC
2rdquo acetyl coenzyme A
(AcCoA) unit but also an intermediate toward aliphatic amino acids and mEP
AcCoA is the building block of fatty acids polyketides and mevalonic acid (mVA) a cytosolic precursor of the C
5 isoprene units for the biosynthesis of terpenes in the
C15
and C30
series (mind it is different from the mEP pathway in product and in cell location) Finally AcCoA enters the citric acid or Krebs cycle which leads to several precursors of amino acids These are oxaloacetic acid precursor of aspartic acid through transamination (thus toward lysine as a nitrogenated C
5N linear unit and methishy
onine as a methyl supplier) and 2‐oxoglutaric acid preshycursor of glutamic acid (and subsequent derivatives such as ornithine as a nitrogenated C
4N linear unit) All these
amino acids are key precursors in the biosynthesis of many alkaloids
114 Reactions Involved in the Construction of Secondary Metabolites
most reactions occurring in the living cells are performed by specialized enzymes which have been classified in an intershynational nomenclature defined by an enzyme commission (EC) number There are six classes of enzymes depending on the biochemical reaction they catalyze EC‐1 oxidoreducshytases (catalyzing oxidoreduction reactions) EC‐2 transfershyases (catalyzing the transfer of functional groups) EC‐3 hydrolases (catalyzing hydrolysis) EC‐4 lyases (breaking bonds through another process than hydrolysis or oxidation leading to a new double bond or a new cycle) EC‐5 isomershyases (catalyzing the isomerization of a molecule) and EC‐6 ligases (forming a covalent bond between two molecules) many subclasses of these enzymes have been described depending on the type of atoms and functional groups involved in the reaction and if any on the cofactor used in this reaction For example several cofactors can be used by dehydrogenases like NAD(P)NAD(P)H FADFADH
2 or
FmNFmNH2 For a description of this classification the
reader can refer to specialized Internet websites like ExplorEnz [1] What is important to realize is that most enzymes are substrate specific and have been selected during
CHOHO
CO2H
O
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
HOOH
HO
HOH2CO
OHC
CH2OPO32ndash
OHHO
CO2H
HO
OH
OH
TYR
PHE
CO2H
NH2
TRP
CH2OPO32ndash
HO
HOH2COH
MEP
C10 C20 and C40 terpenes
CO2HHO
SER
GLY CYS
CO2HOPO3
2ndash
CH2OH
OHHO2C
MVA
C15 and C30 terpenes
Polyketides
Krebscycle
(citric acid)
VAL ALA ILE LEU
CO2H
O
HO2C
CO2HO
CO2H
ASP
LYS
GLU
Glucose-6-phosphate
Fructose-6-phosphate
Erythrose-4-phosphate
3-PGAL
OP2O63ndash OP2O6
3ndash
IPP DMAPP
3-PGA PEP PA
O SCoAAcCoA
3x
Cytosol
Cytosol
Chloroplasts
Chloroplasts
Glycolysis
C2
C5
Shikimic acid Oxaloaceticacid
2-Oxoglutaricacid
MET
THR
PRO
ARGORN
Alkaloids Alkaloids Alkaloids Alkaloids
Aminoacids
Peptidesproteins
Phenolics
C1
C5N C4N
ldquoCH3rdquo
(Indole)C2NC6C3C6C2N C6C1
Pentosephosphatepathway
SCHEME 12 The building block chart involving glycolysis and the Krebs cycle
4 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
evolution to perform specific transformations making natural products with often and yet unknown functions
Secondary metabolites arise from specific biosynthetic pathways which use the previously defined building blocks The bunch of organic reactions involved in these biosynshytheses allows the construction of natural product frameshyworks which are finally diversified through ldquodecorationrdquo steps (Scheme 13) It is not the purpose of this introductive chapter to describe in detail all biosynthetic pathways and the reader can refer to excellent books and articles which have been published elsewhere [2 3]
The reactions involved in the construction of natural product skeletons will be described later for representative classes of compounds The identification of the building block footprint in the natural product skeleton will be emphasized as much as possible sometimes referring to biogenetic speculations [4] After the framework construction the decoration steps will involve as diverse reactions as aliphatic CH oxidations (eg involving a cytochrome P
450 oxygenase) occasionally triggering a
rearrangement heteroatom alkylations (eg methylation by S‐adenosylmethionine) or allylation (by DmAPP) esterifications heteroatom or C‐glycosylations (leading to heterosides) radical couplings (especially for phenols) alcohol oxidations or ketone reductions amineketone transaminations alkene dihydroxylations or epoxidations oxidative halogenations BaeyerndashVilliger oxidations and further oxygenation steps At the end of the biosynthesis such transformations may totally hide the primary building block origin of natural products
115 Secondary Metabolisms
1151 Polyketides Polyketides (or polyacetates) are issued from the oligomerization of C
2 acetate units performed
by polyketide synthases (PKS) and leading to (C2)
n linear
intermediates [5 6] If the (C2)
n intermediates arise from
successive Claisen reactions performed by ketosynthase
domains (KS in nonreducing PKS) a highly reactive poly‐ β‐ketoacyl intermediate H(CH
2CO)
nOH is formed
leading to phenolic and aromatic products through further intramolecular Claisen condensations Furthermore highly reducing PKSs are made of specialized enzymatic subunits working in line or iteratively to functionalize each C
2 linker
bond as CH(OH)CH2 (by ketoreductases (KR)) then as
HCCH (by dehydratases (DH)) and as CH2CH
2 (by enoyl
reductases (ER)) leading to a high degree of functionalization of the final product (Fig 12) By these iterative sequences highly reduced polyketides which can be either linear macrocyclized or polycyclized depending on the reactivity of the biosynthetic intermediates can be formed [7] With the same logic fatty acids are also biosynthesized by fatty acid synthases
moreover the PKS enzyme can be hybridized with nonshyribosomal peptide synthetase (NRPS) domains (see also ldquoNRPS metabolites and peptidesrdquo in the ldquoAlkaloidsrdquo secshytion) leading to the acylation of an amino acid by the (C
2)
n
acyl intermediate As previously the functionalization of the acyl chain depends on the PKS enzyme and the PKSNRPS products are also extremely diversified (eg hirsutellone B Fig 12) [8]
1152 Terpenes Terpenes are derived from the oligoshymerization of the C
5 isoprene units DmAPP and IPP Both
precursors are prompt to generate either an allylic cation (the diphosphate is a good leaving group) or a tertiary carbocation respectively which makes the IPP easy to react with DmAPP (Scheme 14) This reaction happens in the active site of a terpene synthase which activates the deparshyture of the diphosphate group from DmAPP thanks to Lewis acid activation (a metal like mg2+ mn2+ or Co2+ is present in the enzyme active site [9]) This leads to geranyl (C
10
monoterpene precursor) or farnesyl (C15
sesquiterpene precursor) diphosphate depending on the location of the enzyme (chloroplast for the mEP pathway or cytosol for the mVA pathway) Geranylgeranyl (C
20 diterpene) and
Constructionreactions
Decoration reaction(functionalization)Building
blocksNatural product
frameworksNatural products
HH
OH OAcH
HO O OH
OHO
OBz
P450
P450
P450
P450
P450 P450 Oxidativeauxiliaryenzymes
Taxadiene
OP2O63ndash
OP2O63ndash
3times
(a)
(b)
10-Deacetylbaccatin III
DMAPP
IPPand electrophilic
cyclizations
SCHEME 13 (a) From building blocks to natural products and (b) the example of 10‐deacetylbaccatin III
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 5
farnesylfarnesyl (C30
triterpene precursor) diphosphates can also be obtained by further additions of IPP leading to longer linear intermediates
The cyclization of linear precursors is achieved by speshycialized cyclases which generate a poorly functionalized natural product framework [10 11] Auxiliary enzymes such as oxygenases then increase the complexity and the diversity
of compounds by further functionalization (Scheme 13b) [12] A high degree of oxidation can be observed in comshypounds like thapsigargin paclitaxel or bilobalide (Fig 13) The biosynthesis of this last compound for example involves a high oxygenation pattern two Wagnerndashmeerwein rearrangements and several oxidative cleavages leading to the loss of five carbons The resulting natural products can
KS
S-ACP
O O
KR
S-ACP
OH O
S-MAT
O
YesNo
R
R
R
YesNoDH
S-ACP
OR
YesNoER
S-ACP
OR
YesNoTE
OH
OR
New cycle
New cycleor
release
New cycleor
release
New cycleor
release
Release
R in
crem
ente
d by
two
carb
ons
HO
O
S-ACP
O
Claisen condensation (ndashCO2)
reduction (NADPH)
dehydration (ndashH2O)
+
reduction (NADPH)
hydrolysis (H2O)
O O NH
OH
OH
H H
H H
Hirsutellone B(mixed PKSNRPS
product)
OO
O
OHOH
HO OH
Norsolorinic acid
O
O
O
O CHO
OH
HH
H
H
HH
OHHemibrevetoxin B
OOHO
OHO
HOHO OH
Erythronolide A
OH
OH
HO
Panaxytriol
From ahexanoylstartingblock
H
H
Fromtyrosine
H
1C lost fromdecarboxylation
FIGURE 12 Chemical logic of polyketide construction leading to variable functionalization of the elongated acyl chain and examples of resulting chemical diversity ACP acyl carrier protein DH dehydratase ER enoyl reductase KR ketoreductase KS ketosynthase mAT malonyl acyl transferase TE thioesterase
DMAPPOP2O6
3ndashOP2O6
3ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
Geranyl diphosphate (GPP)2 times IPP
H H
Geranylgeranyl diphosphate (GGPP)
Examplesverticillane
casbanetaxane
phorbol
Exampleslabdane
pimaranekauraneabietane
aphidicolanegibberellane
IPP
SCHEME 14 Early assembly of C5 units in terpene biosynthesis leading to diterpenes (C
20)
6 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
thus be extremely modified with structures whose biogenetic origin is far from being obvious at first sight and cannot be determined without further experiments such as isotopic labeling
1153 Flavonoids Resveratrols Gallic Acids and Further Polyphenolics We have previously discussed the polyketide origin of some phenolic compounds based on the (C
2)
n motif Other polyphenols like gallic acids are directly
derived by the aromatization of shikimic acid (C6C
1 building
block Scheme 12) [13] The C6C
3 building blocks are availshy
able from the conversion of phenylalanine and tyrosine into cinnamic and p‐coumaric acids respectively and then by further hydroxylation steps (Scheme 15) These can dimerize into lignans (eg podophyllotoxin) [14 15] through radical processes or converted to low molecular weight compounds like eugenol coumarins or vanillin [16] The coenzyme A thioesters of these C
6C
3 acids can be used as initiator units
by specialized ketosynthases for an elongation by two acetyl units leading to aromatic polyketides like styrylpyrones or diarylheptanoids (eg curcumin) [17] Important comshypounds from this metabolism are flavonoids (C
6C
3C
6) [18]
and stilbenoids (C6C
2C
6) (a decarboxylation occurs during
the aryl cyclization) [19] which are synthesized by chalcone synthase and stilbene synthase respectively Flavonoids (eg catechin) and stilbenes (eg resveratrol) are present in large amounts in fruits and vegetables and may exert their radical scavenging properties in vivo
1154 Alkaloids Alkaloids are nitrogen‐containing compounds The nitrogen(s) can be involved in an amine function conferring basicity to the natural product (like ldquoalkalirdquo) or in less or nonbasic functions such as an amide a nitrile an isonitrile or an ammonium salt (quaternary amines) For amines protonation often occurs at physiological pH and may condition their biological activity In many cases the nitrogen is biogenetically derived from an amino acid We will thus discuss alkaloids according to their amino acid origin
Alkaloids Derived from the Krebs Cycle (Lysine and Ornithine Derived) As shown previously (Scheme 12) the Krebs cycle is a crucial metabolic process which leads to α‐ketoacids (oxaloacetic and 2‐oxoglutaric acids) Their enzymatic transamination affords the two amino acidsmdashaspartic acid and glutamic acidmdashwhich are the direct biosynthetic precursors of amino acids lysine and ornithine respectively These in turn produce cadaverine a ldquoC
5Nrdquo unit
and putrescine a ldquoC4Nrdquo unit which are major components
for the biosynthesis of important alkaloids as will be discussed later (Scheme 16) Additionally ornithine is a precursor of arginine another important amino acid
ornithine‐derived alkaloids (incorporating the c4n
unit) Putrescine is derived from the decarboxylation of ornithine and is a precursor of linear polyamines like spermshyine After enzymatic methylation of one amine of putrescine
C10 C15
C15
C10
C10
C30
CO2H
Chrysanthemic acid
OH
Menthol
O
HO
H
HOGlc
MeO2CLoganin (iridoid)
H
H
H
HH
OO
OParthenolide
O
O
OHOHO
OAcHO
OnC3H7
O
O
O
nC7H15
Thapsigargin
O
O
OO
H
H
O
Cleavage
H
H
Artemisinin
C20 C40
O
O
OO
OO
OHOHH
Bilobalide(highly rearrangedand missing 5 C)
CleavageH
Highoxidativecleavages
HO OAcH
AcO O OH
OOOBz
O
Ph
BzNH
OHFrom PHE Paclitaxel
C25O
H
HO
OHCH
OH Ophiobolin A
H
HO
H
H
HHCholesterol
(missing 3 C WM)
H
H H H
H
H
Hopene
β-Carotene
C30 - 3
C15
C20 - 5WM
WM
FIGURE 13 Chemical diversity in the terpene series (Wm Wagnerndashmeerwein shifts lost carbons bold bonds are remnant of primary building blocks)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 7
in the presence of S‐adenosylmethionine transamination of the other affords γ‐(N‐methylamino)aldehyde [20] The resulting cyclic iminium is a key intermediate in the formation of many medicinally important alkaloids such as
the plant‐derived compounds cocaine atropine or the calysshytegines [21 22] Indeed this iminium is a mannich acceptor which can react with various nucleophiles the first of those being the carbanion of acetyl‐CoA Thus after a stepwise
CO2H
RCinnamic acid (R=H)p-Coumaric acid (R=OH)
RO
PHETYR
OH
[H] [O]
lignans
OH
O
O
O
O
OMeMeO
OMe
C mdash C andor C mdash Oradical couplings
After E Zisomerization
for example Podophyllotoxin
O ORO
Coumarins(eg scopoletin)
[O]
Prenylationcyclizationcleavage[O]
O OO
Furocoumarins(eg psoralen)
RO
nAcetyl-CoA
thencyclization
n = 2 styrylpyrones
O O
OMe
n = 3 avonoids(from chalcone synthase)
C6C3
H
H H
From AcCoA
O
n = 3 stilbenoids(from stilbene synthase)
HO
OH
OHFrom AcCoA From AcCoA(ndashCO2)
Resveratrol
HO
OH
OH
OH
OH
H
Catechin
MeO Yangonin
From AcCoA
SCHEME 15 The phenylpropanoid biosynthetic pathways
LYS
CO2
Cadaverine
O NH
H2O
NH
Piperideineiminium
Pelletierine
AcAcCoA
CO2
NH
O
N
O
Pseudopelletierine
NH
Ph
OH
Ph
O
Lobeline
NH
δ+
δndash
N
H
H
N
H
OH
Lupinine
N
H N
H
(+)-Sparteine
N
NH
O (ndash)-Cytisine
NH
CO2H
Pipecolic acid
N
OHHO
OH
HO
H
Castanos permine
ORN
CO2
NH2 NH2 NH2 NH2
NH2
NH2
NH2
PutrescineO
NHMe
N-Methylpyrrolinium
2 timesAcCoA
NMe
ARG
n = 1 spermidinen = 2 homospermidine
NMe
O
HygrineCO2
CO2
MeN
O
[O]
TropinoneCO2
HNHO
OHOH
OH
Calystegine B2
MeN
OAtropine
O
Ph
HO
NMe
NMe
NMe
O
Cuscohygrine
HN
HN
NH2
( )n
O
n = 2
N
HHO
Retronecine
HH
H
H
H
HO
H H
H
SCHEME 16 Lysine‐ and ornithine‐derived alkaloid biosynthetic pathways (mind the structural similarities)
8 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
elongation by two AcCoA units either decarboxylation can occur leading to the acetonylpyrrolidine hygrine or a secshyond mannich reaction by the intramolecular attack of the acetoacetate anion onto an oxidation‐derived pyrrolinium leading to the tropane skeleton (tropinone) The acetoacetyl‐CoA intermediate can also react intermolecularly with another pyrrolinium cation leading to cuscohygrine after decarboxylation Finally the pyrrolizidine alkaloids [23] are derived from homospermidine which when submitted to terminal oxidative deamination leads to the bicyclic skelshyeton of retronecine and further Senecio alkaloids We can mention herein that ornithine is a biosynthetic precursor of arginine bearing a guanidine function which is an intermediate toward the toxic compounds tetrodotoxin and saxitoxin (not shown)
lysine‐derived alkaloids (incorporating the c5n
unit) From lysine to piperidine alkaloids the biosynthetic steps parallel the one previously described from ornithine Indeed the oxidative deamination of cadaverine affords a δ‐amino aldehyde which cyclizes through imine formation into piperideine Protonation results in a mannich acceptor which is able to react with various nucleophiles such as β‐ketothioester anions The first product of these reactions is pelletierine which can further react through an intramolecular mannich
reaction leading to pseudopelletierine Quinolizidines [24] can also be formed first from the mannich reaction of the piperideine acceptor with the corresponding enamine nucleoshyphile and then after additional transformation steps leading for example to lupinine sparteine or cytisine
Indolizidine alkaloids [15] such as castanospermine and swainsonine are formed from pipecolic acid an amino acid derived from lysine which can be elongated by malonyl‐CoA followed by ring closure When protonated these alkaloids are oxonium mimics strongly inhibiting glycosidases
Tyrosine‐ and Phenylalanine‐Derived Alkaloids Tyrosine and phenylalanine amino acids are bearing the phenylshyethylamine moiety of many medicinally relevant alkaloids Further hydroxylations on the aromatic carbocycle or on the aliphatic part can be observed methylations can occur on phenolic oxygens and on the amine leading to catecholamines (adrenaline noradrenaline dopamine) Arylethylamines are also usual to react with endogenous aldehydes through PictetndashSpengler reactions [25] leading to important biosynthetic intermediates (Scheme 17) like
bull Reticuline from the reaction with 4‐hydroxyphenylacetshyaldehyde toward benzyltetrahydroisoquinoline alkaloids
NH2
Phenylethylamines
RONH TetrahydroisoquinolinesRO
RʹCHO
Rʹ
NH
(CH2)n
MeO
HO
Benzyltetrahydroisoquinoline(n = 1) for example reticuline
orPhenethyltetrahydroisoquinoline
(n = 2) eg autumnaline
IntermolecularCmdashO phenol
couplings
Curarealkaloids
IntramolecularC mdash C phenol
couplings
ONMe
HHO
H
HO
Morphine
NMe
MeO
HO
MeOOH
Isoboldine
O
O
OMe
NO2
CO2H
Aristolochic acid
OMe
O
NHAcMeO
MeOMeO
Colchicine
N
O
O
OMe
OMeBerberine
IntramolecularCmdash C coupling
and furtheroxidative events
Rʹ derivedfrom PHE
Pictet-Spenglerreaction
TYR
HO
HO CHO
HNHO
HO
OH
OMeO
OH
NMe
[O]
GalantamineNorbelladine
Rʹ derived fromsecologanin
NAc
HO
HO
O
CO2MeH
H
H
OHIpecosideaglycone
Ipecac alkaloids
1ClostCmdash C cleavage
and ring extension
H
ROH
1C lost
Cleavage
SCHEME 17 Tyrosine‐derived alkaloid biosynthetic pathways (double head arrows figure bond cleavages during biosynthetic processes)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 9
morphine berberine tubocurarine isoboldine or the highly modified aristolochic acid [26 27]
bull Automnaline from the reaction with 3‐(4‐hydroxyphenyl)propanal toward phenylethyltetrahydroisoquinoline alkaloids colchicine cephalotaxine or schelhammerishycine [28 29]
bull Ipecoside from the reaction of dopamine with secoloshyganin toward terpene tetrahydroisoquinoline alkaloids ipecoside or emetine
Lastly norbelladine (top of Scheme 17) is issued from the reductive amination of 34‐dihydroxybenzaldehyde (derived from phenylalanine) with tyramine (derived from tyrosine) and constitutes a biosynthetic node leading to Amaryllidaceae alkaloids such as galantamine crinine or lycorine depending on the topology of phenolic couplings In all these biosynthetic routes radical phenolic couplings are key reactions for CC and CO bond formations and rearrangements [30 31]
Tryptophan‐Derived Indole and Indole Monoterpene Alkaloids As for alkaloids derived from tyrosine and phenylalanine those derived from tryptophan are formed after decarboxylation of the amino acid (into tryptamine) and possible hydroxylation of the aromatic carbocycle (eg serotonin) and N‐methylation (eg psilocin) As previously tryptamine can also react through PictetndashSpengler reactions to form tetrahydro‐β‐carbolines which can be aromatized for example into harmine (Scheme 18) [16]
When the aldehyde partner of the PictetndashSpengler reacshytion with tryptamine is the terpene secologanin strictosidine is formed as an entry toward the vast monoterpene indole alkaloids [32 33] Hydrolysis of the glucosidic part releases the strictosidine aglycone bearing an aldehyde while iminshyium formation and further cyclization and reduction can lead to ajmalicine (from oxocyclization) or yohimbine (from carshybocyclization) These alkaloids are referred to as from the Corynanthe type with the monoterpene carbon skeleton unmodified Although it misses one carbon and has a very
RO
Carbonylcompounds
TRPNH
NH2
Indolethylamines(eg serotonine)
RONH
NH
Rʹ NH
N
MeOPictet-Spenglerreaction for example Harmineβ-Carbolines
NH
NHH
O
MeO2C
MeO2C
OH
H
H
Rʹ derived from secologanin
Strictosidine aglycone
NH
N
OH
H
H
Ajmalicine
N
N
O
O
H
H
H
Strychnine (C lost)Corynanthe type
Aspidosperma type Iboga type
N
N
OH
OMe
Quinine (C lost)
N
NH CO2Me
Catharanthine
NH
N
CO2MeH
OAc
OH
Vindoline
Complextransformations
NH
NMe
HN
MeN
H
H
Chimonanthine
Cyclizationdimerization
Prenylationcyclization
NH
NMeHO2C
H
Lysergic acid
H
H
Ergot alkaloidsPyrroloindoles Indole monoterpene
1C lost
1C lostFrom AcCoA
SCHEME 18 Tryptophan‐derived alkaloid biosynthetic pathways (gray parts monoterpenic units)
10 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
different structure strychnine is related to the Corynanthe alkaloids incorporating two carbons from acetyl‐CoA Highly modified monoterpene skeletons are derived from the Corynanthe core through CC bond breaking and reorshyganization leading to Iboga‐type (eg catharanthine) and Aspidosperma‐type (eg vindoline) alkaloids The antishycancer drug vinblastine is a heterodimer resulting from the nucleophilic attack of vindoline on a mannich acceptor resulting from catharanthine found in madagascar perishywinkle (Catharanthus roseus) The heteroaromatic comshypounds ellipticine camptothecin and quinine are also derived from a Corynanthe‐type precursor although in this case the biosynthetic relationship may not be obvious due to deep modifications of the skeleton
Finally two important classes of compounds have to be mentioned since they have inspired many synthetic chemists The pyrroloindole alkaloids result from the cyclization of tryptamine as found in physostigmine (formed by a cationic mechanism after methylation in position 3 of the indole not shown) or in chimonanthine (presumably formed by a radical coupling mechanism Scheme 18) The ergot alkaloids are derived from the 33‐dimethylallylation on position 4 of the indole in trypshytophan whose further cyclization and oxidation processes afford the natural products (eg lysergic acid Scheme 18 and ergotamine) which have had important medical applications [34]
NRPS Metabolites and Peptides NRPS enzymes assemble amino acids including nonproteinogenic ones into oligopeptides The enzymes contain several modules and especially an adenylation domain (A) which specifically selects and activates the amino acid to be transferred as a thioester on the nearby peptidyl carrier protein (PCP) [2] A condensation module (C) then catalyzes the formation of the peptide bonds between the newly introduced amino acyl‐PCP (bearing a free amine) and the elongated peptidyl‐PCP thioester At the end of the elongation a cyclization can occur into cyclopeptides but the peptide can also be
transferred to auxiliary enzymes like methyltransferases glycosyltransferases or oxidases (vancomycins are typical products of such functionalizations) [35 36] The formation of heterocycles is also frequently encountered in this metabolism as in penicillins that are derived from the tripeptide α‐aminoadipoyl‐cysteinyl‐valine or telomestatin (Fig 14) [2]
Other Alkaloid Origins There are many other nitrogen sources involved in alkaloid biosyntheses for example nicotinic acid (originated from aspartic acid and intershymediate in nicotine and anabasine biosyntheses) and anthranilic acid (originated from tryptophan and intermedishyate toward acridines or aurachins) The amination reaction (eg through transamination of carbonyl compounds) is also a way to introduce nitrogens in natural products for example from fatty acids steroids (toward Solanum alkaloids or cyclopamines) or other terpenoids (aconitine and atisine have diterpene skeletons while Daphniphyllum alkaloids are triterpene derivatives) Finally nucleic acids can also be precursors of alkaloids like the well‐known caffeine
12 FROM BIOSYNTHESIS TO TOTAL SYNTHESIS STRATEGIES TOWARD THE NATURAL PRODUCT CHEMICAL SPACE
121 The Chemical Space of Natural Products
Natural products occupy an important place in human com munities as demonstrated by their vast use from ancient times to nowadays like dyes fibers oils pershyfumes agrochemicals or drugs Broadly both primary and secondary metabolites could be classified as natural products while the latter as discussed previously are usushyally regarded as the ldquonatural productsrdquo owing to their comshyplexity and diversity arising from a variety of biosynthetic pathways The structural chemical diversity found among
N
S
CO2HO
HNPhH2C
O
Penicillin G
OH
HN
HN
O
O
ONH2
O
OHO
NHMe
OCl
O
NH
NH
NH
O
ClHO
O
HN
H
H
HOOH
OHHO2C
Vancomycin aglycone
ON
NO
O
NN
O
O
N
N O
Me
S
N N
OMeH
Telomestatin
L-AAA-L-CYS-D-VAL
Enzymes
FIGURE 14 Structural diversity of nonribosomal peptide compounds (AAA α‐aminoadipic acid)
FROm BIOSyNTHESIS TO TOTAL SyNTHESIS STRATEGIES TOWARD THE NATuRAL PRODuCT CHEmICAL SPACE 11
all living organisms defining the chemical space of natural products [37] is the consequence of their evolution occurshyring as an adaptation of organisms to their environment It is commonly believed that secondary metabolites are proshyduced as messengers by living organisms or as weapons against enemies and thus they should have certain biological activities in a medicinal point of view [38] Indeed natural products are regarded as one of the main sources of medicines (Fig 15) From the traditional medicinal extracts to every single bioactive molecule the methods of extraction purification identification and biological investigation of natural products have been well established Their complex structures and interesting properties have attracted synthetic chemists to accomplish their total syntheses and that of medicinally relevant anashylogs sometimes in the industrial context [39] Thus tarshygeting the chemical space of natural products has never been more relevant than today Although the discovery of natural products demands time and labor‐consuming manipulations it is worth to notice that the knowledge on this chemical space is still continually growing while biological advances allow for discovering and undershystanding potential targets However increasing the chemical space of human‐made compounds based on natural products should benefit from transdisciplinary colshylaborations such as the use of coupled biosynthetic and chemical synthetic methods to design original ldquounnatural natural productsrdquo [40]
122 The Biosynthetic Pathways as an Inspiration for Synthetic Challenges
1221 Precursor‐Directed Biosyntheses and Mutashysynthetic Strategies to Increase the Chemical Space of Natural Products As the genetics and biochemistry of natural product biosynthesis are better understood novel biosynthetic techniques have been developed to study and generate new diversity in natural product analogs (Scheme 19) Precursor‐directed biosynthesis (PDB) is considered as the earliest example of combining chemical and biological methods for the generation of complex natural product analogs [41 42] This approach compared with the biosynthetic pathway of wild‐type metabolites (Scheme 19a) involves the feeding of analogs of the natural biosynthetic building blocks to the living organisms (Scheme 19b) usually bacteria or fungi which incorporate the modified precursors into the biosynthesized compound mutasynthesis also termed as mutational biosynthesis (mBS) involves the inactivation of a key step of the bioshysynthesis in a mutant microorganism (Scheme 19c) which can then be fed by various modified or advanced building blocks (mutasynthons Scheme 19d) [43] These mutasshyynthons could not be incorporated by the wild type due to specificities of the enzymatic machinery Build up on PDB mBS eliminates the natural biosynthetic intermediate thus generating a less complex mixture of metabolites and making the purification or yield of target products better
O NH
OHO
O
OO
O
O
OH
HOO
O O
OH
HOO
NH ONH2
O
ONH
NH
O
OCl Cl
O
O
OH
HN
HN
HN
HN
OO
HO
HN
OH
OHOH
O
O
HO
HO
OHO
O
OHH2N
O
OH3C
H
O
H
CH3
CH3
H
O O
NO
H
O
HO
O
NO
O
NH
HN
OHO
HONH
ON
H
HO
O
H2N
O OH
ONH
OH
ON OHO
OH
HO OS OH
OOOH
OHO
O
ON
OO
O
HO
O
O OH
OO
Micafunginantifungal
Rapamycinimmunosuppressive
Galantamineanti-Alzheimer
Taxolanticancer
Artemisininantimalarial
Vancomycinantibiotic
FIGURE 15 Some famous natural products currently used as drugs
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product
Dedicated to my mother father and wife
LIST OF CONTRIBUTORS xiii
PREFACE xv
1 From Biosyntheses to Total Syntheses An Introduction 1Bastien Nay and Xu‐Wen Li
11 From Primary to Secondary Metabolism the Key Building Blocks 1111 Definitions 1112 Energy Supply and Carbon Storing at the Early Stage
of Metabolisms 1113 Glucose as a Starting Material toward Key Building Blocks
of the Secondary Metabolism 1114 Reactions Involved in the Construction of Secondary Metabolites 3115 Secondary Metabolisms 4
12 From Biosynthesis to total Synthesis Strategies toward the Natural Product Chemical Space 10121 the Chemical Space of Natural Products 10122 the Biosynthetic Pathways as an Inspiration
for Synthetic Challenges 11123 the Science of total Synthesis 14124 Conclusion a Journey in the Future of total Synthesis 16
References 16
SECTION I ACETATE BIOSYNTHETIC PATHWAY 19
2 Polyketides 21Franccediloise Schaefers Tobias A M Gulder Cyril Bressy Michael Smietana Erica Benedetti Stellios Arseniyadis Markus Kalesse and Martin Cordes
21 Polyketide Biosynthesis 21211 Introduction 21212 assembly of acetateMalonate‐Derived Metabolites 23213 Classification of Polyketide Biosynthetic Machineries 23214 Conclusion 39
References 40
CONTENTS
viii CoNtENtS
22 Synthesis of Polyketides 44221 asymmetric alkylation Reactions 44222 applications of asymmetric alkylation Reactions in total Synthesis
of Polyketides and Macrolides 60References 8323 Synthesis of Polyketides‐Focus on Macrolides 87
231 Introduction 87232 Stereoselective Synthesis of 13‐Diols asymmetric aldol Reactions 88233 Stereoselective Synthesis of 13‐Diols asymmetric Reductions 106234 application of Stereoselective Synthesis of 13‐Diols in
the total Synthesis of Macrolides 117235 Conclusion 126
References 126
3 Fatty Acids and Their Derivatives 130Anders Vik and Trond Vidar Hansen
31 Introduction 13032 Biosynthesis 130
321 Fatty acids and Lipids 130322 Polyunsaturated Fatty acids 134323 Mediated oxidations of ω‐3 and ω‐6 Polyunsaturated
Fatty acids 13533 Synthesis of ω‐3 and ω‐6 all‐Z Polyunsaturated Fatty acids 140
331 Synthesis of Polyunsaturated Fatty acids by the Wittig Reaction or by the Polyyne Semihydrogenation 140
332 Synthesis of Polyunsaturated Fatty acids via Cross Coupling Reactions 143
34 applications in total Synthesis of Polyunsaturated Fatty acids 145341 Palladium‐Catalyzed Cross Coupling Reactions 145342 Biomimetic transformations of Polyunsaturated Fatty acids 149343 Landmark total Syntheses 153344 Synthesis of Leukotriene B
5 158
35 Conclusion 160acknowledgments 160References 160
4 Polyethers 162Youwei Xie and Paul E Floreancig
41 Introduction 16242 Biosynthesis 162
421 Ionophore antibiotics 162422 Marine Ladder toxins 165423 annonaceous acetogenins and terpene Polyethers 165
43 Epoxide Reactivity and Stereoselective Synthesis 166431 Regiocontrol in Epoxide‐opening Reactions 166432 Stereoselective Epoxide Synthesis 172
44 applications to total Synthesis 176441 acid‐Mediated transformations 176442 Cascades via Epoxonium Ion Formation 179443 Cyclizations under Basic Conditions 181444 Cyclization in Water 182
45 Conclusions 183References 184
CoNtENtS ix
SECTION II MEVALONATE BIOSYNTHETIC PATHWAY 187
5 From Acetate to Mevalonate and Deoxyxylulose Phosphate Biosynthetic Pathways An Introduction to Terpenoids 189Alexandros L Zografos and Elissavet E Anagnostaki
51 Introduction 18952 Mevalonic acid Pathway 19153 Mevalonate‐Independent Pathway 19254 Conclusion 194References 194
6 Monoterpenes and Iridoids 196Mario Waser and Uwe Rinner
61 Introduction 19662 Biosynthesis 196
621 acyclic Monoterpenes 197622 Cyclic Monoterpenes 197623 Iridoids 200624 Irregular Monoterpenes 202
63 asymmetric organocatalysis 203631 Introduction and Historical Background 204632 Enamine Iminium and Singly occupied Molecular
orbital activation 207633 Chiral (Broslashnsted) acids and H‐Bonding Donors 213634 Chiral BroslashnstedLewis Bases and Nucleophilic Catalysis 218635 asymmetric Phase‐transfer Catalysis 220
64 organocatalysis in the total Synthesis of Iridoids and Monoterpenoid Indole alkaloids 225641 (+)‐Geniposide and 7‐Deoxyloganin 226642 (ndash)‐Brasoside and (ndash)‐Littoralisone 227643 (+)‐Mitsugashiwalactone 229644 alstoscholarine 229645 (+)‐aspidospermidine and (+)‐Vincadifformine 230646 (+)‐Yohimbine 230
65 Conclusion 231References 231
7 Sesquiterpenes 236Alexandros L Zografos and Elissavet E Anagnostaki
71 Biosynthesis 23672 Cycloisomerization Reactions in organic Synthesis 244
721 Enyne Cycloisomerization 245722 Diene Cycloisomerization 257
73 application of Cycloisomerizations in the total Synthesis of Sesquiterpenoids 266731 Picrotoxane Sesquiterpenes 266732 aromadendrane Sesquiterpenes Epiglobulol 267733 CubebolndashCubebenes Sesquiterpenes 267734 Ventricos‐7(13)‐ene 270735 Englerins 271736 Echinopines 271737 Cyperolone 273
x CoNtENtS
738 Diverse Sesquiterpenoids 27674 Conclusion 276References 276
8 Diterpenes 279Louis Barriault
81 Introduction 27982 Biosynthesis of Diterpenes Based on Cationic Cyclizations
12‐Shifts and transannular Processes 27983 Pericyclic Reactions and their application in the Synthesis
of Selected Diterpenoids 284831 Dielsndashalder Reaction and Its application in the total
Synthesis of Diterpenes 284832 Cascade Pericyclic Reactions and their application in the total
Synthesis of Diterpenes 29184 Conclusion 293
References 294
9 Higher Terpenes and Steroids 296Kazuaki Ishihara
91 Introduction 29692 Biosynthesis 29693 Cascade Polyene Cyclizations 303
931 Diastereoselective Polyene Cyclizations 303932 ldquoChiral proton (H+)rdquo‐Induced Polyene Cyclizations 304933 ldquoChiral Metal Ionrdquo‐Induced Polyene Cyclizations 308934 ldquoChiral Halonium Ion (X+)rdquo‐Induced Polyene Cyclizations 313935 ldquoChiral Carbocationrdquo‐Induced Polyene Cyclizations 319936 Stereoselective Cyclizations of Homo(polyprenyl)arene
analogs 31994 Biomimetic total Synthesis of terpenes and Steroids through
Polyene Cyclization 31995 Conclusion 328
References 328
SECTION III SHIKIMIC ACID BIOSYNTHETIC PATHWAY 331
10 Lignans Lignins and Resveratrols 333Yu Peng
101 Biosynthesis 3331011 Primary Metabolism of Shikimic acid and aromatic
amino acids 3331012 Lignans and Lignin 335
102 auxiliary‐assisted C(sp3)ndashH arylation Reactions in organic Synthesis 336
103 FriedelndashCrafts Reactions in organic Synthesis 344104 total Synthesis of Lignans by C(sp3)H arylation Reactions 353105 total Synthesis of Lignans and Polymeric Resveratrol by
FriedelndashCrafts Reactions 357106 Conclusion 375References 375
CoNtENtS xi
SECTION IV MIXED BIOSYNTHETIC PATHWAYSndash THE STORY OF ALKALOIDS 381
11 Ornithine and Lysine Alkaloids 383Sebastian Brauch Wouter S Veldmate and Floris P J T Rutjes
111 Biosynthesis of l‐ornithine and l‐Lysine alkaloids 3831111 Biosynthetic Formation of alkaloids
Derived from l‐ornithine 3831112 Biosynthetic Formation of alkaloids
Derived from l‐Lysine 388112 the asymmetric Mannich Reaction in organic Synthesis 392
1121 Chiral amines as Catalysts in asymmetric Mannich Reactions 3941122 Chiral Broslashnsted Bases as Catalysts in asymmetric
Mannich Reactions 3981123 Chiral Broslashnsted acids as Catalysts in asymmetric
Mannich Reactions 4041124 organometallic Catalysts in asymmetric Mannich Reactions 4081125 Biocatalytic asymmetric Mannich Reactions 413
113 Mannich and Related Reactions in the total Synthesis of l‐Lysine‐ and l‐ornithine‐Derived alkaloids 414
114 Conclusion 426References 427
12 Tyrosine Alkaloids 431Uwe Rinner and Mario Waser
121 Introduction 431122 Biosynthesis of tyrosine‐Derived alkaloids 431
1221 Phenylethylamines 4311222 Simple tetrahydroisoquinoline alkaloids 4331223 Modified Benzyltetrahydroisoquinoline alkaloids 4331224 Phenethylisoquinoline alkaloids 4361225 amaryllidaceae alkaloids 4381226 Biosynthetic overview of tyrosine‐Derived alkaloids 442
123 arylndasharyl Coupling Reactions 4421231 Copper‐Mediated arylndasharyl Bond Forming Reactions 4431232 Nickel‐Mediated arylndasharyl Bond Forming Reactions 4461233 Palladium‐Mediated arylndasharyl Bond Forming Reactions 4471234 transition Metal‐Catalyzed Couplings of Nonactivated
aryl Compounds 450124 Synthesis of tyrosine‐Derived alkaloids 456
1241 Synthesis of Modified Benzyltetrahydroisoquinoline alkaloids 4561242 Synthesis of Phenethylisoquinoline alkaloids 4601243 Synthesis of amaryllidaceae alkaloids 462
125 Conclusion 468References 469
13 Histidine and Histidine‐Like Alkaloids 473Ian S Young
131 Introduction 473132 Biosynthesis 473133 atom Economy and Protecting‐Group‐Free Chemistry 480
xii CoNtENtS
134 Challenging the Boundaries of Synthesis PIas 488135 Conclusion 497References 499
14 Anthranilic AcidndashTryptophan Alkaloids 502Zhen‐Yu Tang
141 Biosynthesis 502142 Divergent SynthesisndashCollective total Synthesis 508143 Collective total Synthesis of tryptophan‐Derived alkaloids 510
1431 Monoterpene Indole alkaloids 5101432 Bisindole alkaloids 512
References 517
15 Future Directions of Modern Organic Synthesis 519Jakob Pletz and Rolf Breinbauer
151 Introduction 519152 Enzymes in organic Synthesis Merging total
Synthesis with Biosynthesis 520153 Engineered Biosynthesis 526154 Diversity‐oriented Synthesis Biology‐oriented Synthesis
and Diverted total Synthesis 5331541 Diversity‐oriented Synthesis 5351542 Biology‐oriented Synthesis 5361543 Diverted total Synthesis 539
155 Conclusion 541References 545
INDEX 548
Elissavet E Anagnostaki Department of Chemistry Laboratory of Organic Chemistry Aristotle University of Thessaloniki Thessaloniki Greece and Research and Development Department Pharmathen SA Thessaloniki Greece
Stellios Arseniyadis School of Biological and Chemical Sciences Queen Mary University of London London United Kingdom
Louis Barriault Department of Chemistry University of Ottawa Ottawa Ontario Canada
Erica Benedetti Laboratoire de Chimie et Biochimie et Pharmacologiques et Toxicologiques CNRS-Universiteacute Paris Descartes Faculteacute des Sciences Fondamentales et Biomeacutedicales Paris France
Sebastian Brauch Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
Rolf Breinbauer Institute of Organic Chemistry Technische Universitaumlt Graz Graz Austria
Cyril Bressy Aix Marseille Universiteacute Centrale Marseille CNRS Marseille France
Martin Cordes Institute for Organic Chemistry and Center of Biomolecular Drug Research (BMWZ) Leibniz Universitaumlt Hannover Hannover Germany and Helmholtz Center for Infection Research (HZI) Hannover Germany
Paul E Floreancig Department of Chemistry Chevron Science Center University of Pittsburgh Pittsburgh PA USA
Tobias A M Gulder Department of Chemistry and Center for Integrated Protein Science Munich (CIPSM) Biosystems Chemistry Technische Universitaumlt Muumlnchen Munich Germany
Trond Vidar Hansen School of Pharmacy University of Oslo Oslo Norway
Kazuaki Ishihara Department of Biotechnology Graduate School of Engineering Nagoya University Nagoya Japan
Markus Kalesse Institute for Organic Chemistry and Center of Biomolecular Drug Research (BMWZ) Leibniz Universitaumlt Hannover Hannover Germany and Helmholtz Center for Infection Research (HZI) Hannover Germany
Xu‐Wen Li Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
Bastien Nay Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France
Yu Peng State Key Laboratory of Applied Organic Chemistry Lanzhou University Lanzhou China
Jakob Pletz Institute of Organic Chemistry Technische Universitaumlt Graz Graz Austria
Uwe Rinner Institute of Organic Chemistry Johannes Kepler University Linz Linz Austria and Department of Chemistry College of Science Sultan Qaboos University Muscat Oman
Floris P J T Rutjes Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
LIST oF CoNTRIBUToRS
xiv LIST OF CONTRIBUTORS
Franccediloise Schaefers Department of Chemistry and Center for Integrated Protein Science Munich (CIPSM) Biosystems Chemistry Technische Universitaumlt Muumlnchen Munich Germany
Michael Smietana Institut des Biomoleacutecules Max Mousseron CNRS Universiteacute de Montpellier ENSCM France
Zhen‐Yu Tang Department of Pharmaceutical Engineering College of Chemistry and Chemical Engineering Central South University Changsha China
Wouter S Veldmate Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
Anders Vik School of Pharmacy University of Oslo Oslo Norway
Mario Waser Institute of Organic Chemistry Johannes Kepler University Linz Linz Austria
Youwei Xie Max‐Planck‐Institut fuumlr Kohlenforschung Muumllheim Germany
Ian S Young Bristol‐Myers Squibb Company Chemical Development New Brunswick NJ USA
Alexandros L Zografos Department of Chemistry Laboratory of Organic Chemistry Aristotle University of Thessaloniki Thessaloniki Greece
There is pleasure in the pathless woodsthere is rapture in the lonely shore
there is society where none intrudesby the deep sea and music in its roar
I love not Man the less but Nature moreLord Byron
Preface
The first time I came across with the idea of editing a book that merges selected chapters of biosynthesis and total synthesis was when I was teaching postgraduate courses of natural product synthesis at Aristotle University of Thessaloniki This period I realized that the best way to teach youngsters synthesis was to start from the very origin of inspiration nature and its tools biosynthesis
Over the last decades biosynthesis is filling our gaps of understanding the complex mechanisms of nature and pro-vides useful sources of inspiration not only in the way natural products can be synthesized but also by directing synthetic chemists in developing atom‐economical efficient synthetic methods Several are the examples that mimic biosynthetic guidelines from modern iterative alkylations and aldol reactions to CH oxidations that compile nowadays the modern toolbox of organic synthesis
The handed book is constructed in the logic of presenting the parallel development of biosynthesis and organic meth-odology and how these can be applied in efficient syntheses of natural products The book is divided into four sections each representing the four major biosynthetic pathways of natural products namely acetate mevalonate shikimate biosynthetic pathways and the mixed biosynthetic pathways
of alkaloids These sections are divided into chapters that represent selected classes of natural products for example lipids sesquiterpenoids lignans etc Each of these chapters is further divided into three distinct subchapters (a) biosyn-thesis (b) methodological section and (c) application of the described methodology in the total synthesis of the described family of natural products By this way the readers can be focused in the direct comparison between biosyn-theses and the developed methodologies to construct the crucial for each class of natural product carbon bonds Although the book as it develops is focused on presenting the power of biosynthesis and how this power can be applied in providing inspiration for the efficient synthesis of natural products it was not the authors will to present only biomi-metic total syntheses but rather to exploit the modern synthetic methodologies and recognize their disabilities for further improvement
Of course this book will not have been realized without the excellent work of renowned scientists worldwide working either in the field of biosynthesis or total synthesis who collected the existing knowledge on biosynthesis analyzed the existing modern methodologies and presented a bouquet of selected total syntheses Throughout our
xvi PrEfACE
endeavor to complete this book I learned many things from their expertise but I also realized that only with tight collab-orations you can build long‐lasting friendships I would like to thank them all once again for their trust and effort to complete this book We all hope that the current work will contribute to a better understanding of the current status of
organic chemistry and to the discovery of novel strategies and tactics for the synthesis of natural products
Alexandros L ZografosSeptember 2015
Thessaloniki Greece
From Biosynthesis to Total Synthesis Strategies and Tactics for Natural Products First Edition Edited by Alexandros L Zografoscopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
11 FROM PRIMARY TO SECONDARY METABOLISM THE KEY BUILDING BLOCKS
111 Definitions
The primary and secondary metabolisms are traditionally distinguished by their distribution and utility in the living organism network Primary metabolites include carbohyshydrates lipids nucleic acids and proteins (or their amino acid constituents) and are shared by all living organisms on Earth They are transformed by common pathways which are studied by biochemistry (Fig 11) Secondary metabo-lites are structurally diverse compounds usually produced by a limited number of organisms which synthesize them for a special purpose like defense or signaling through specific biosynthetic pathways They are studied by natural product chemistry This distinction is not always so obvious and some compounds can be studied in the context of both primary and secondary metabolisms This is especially true nowadays with the use of genetic and biomolecular tools which tend to make natural product sciences more and more integrative However an important point to remember is that the primary metabolism furnishes key building blocks to the secondary metabolism It would be difficult to describe in detail the full biosynthetic pathshyways in this section We tried to organize the discussion as a vade mecum synthetically gathering information from extremely useful sources which will be cited at the end of this chapter
112 Energy Supply and Carbon Storing at the Early Stage of Metabolisms
The sunlight is essential to life except in some part of the deep oceans It provides energy for plant photosynthesis that splits molecules of water into protons and electrons and releases O
2 (Scheme 11) A proton gradient inside the plant
chloroplasts then drags a transmembrane ATP synthase comshyplex that produces adenosine triphosphate (ATP) while elecshytrons released from water are transferred to the coenzyme reducer nicotinamide adenine dinucleotide phosphate hydride (NADPH) A major function of chloroplasts is to fix CO
2 as a combination to ribulose‐15‐bisphosphate (RuBP)
performed by RuBP carboxylase (rubisco) forming an instable ldquoC
6rdquo β‐ketoacid This is cleaved into two molecules
of 3‐phosphoglycerate (3‐PGA) which is then reduced into 3‐phosphoglyceraldehyde (3‐PGAL a ldquoC
3rdquo triose phosshy
phate) during the Calvin cycle This is one of the major metabolites in the biosynthesis of carbohydrates like glucose and a biochemical mean for storing and retaining carbon atoms in the living cells
113 Glucose as a Starting Material Toward Key Building Blocks of the Secondary Metabolism
Glucose‐6‐phosphate arises from the phosphorylation of glucose It is the starting material of glycolysis an important process of the primary metabolism which consists in eight enzymatic reactions leading to pyruvic acid (PA)
FROM BIOSYNTHESES TO TOTAL SYNTHESES AN INTRODUCTION
Bastien Nay1 and Xu‐Wen Li2
1
1 Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France2 Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
2 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
(Scheme 12) Important intermediates for the secondary metabolism are produced during glycolysis Glucose glucose‐6‐phosphate and fructose‐6‐phosphate can be converted to other hexoses and pentoses that can be oligoshymerized and enter in the composition of heterosides Additionally fructose‐6‐phosphate connects the pentose
phosphate pathway leading to erythrose‐4‐phosphate toward shikimic acid which is a key metabolite in the biosynthesis of aromatic amino acids (phenylalanine tyrosine or C
6C
3
units) and C6C
1 phenolic compounds The next important
intermediate in glycolysis is 3‐PGAL which can be redishyrected toward methylerythritol‐4‐phosphate (mEP) in the
Primary metabolism Secondary metabolism
The field ofbiochemistry
The field ofnatural product
chemistry
Essential toliving organisms
Essential to theproducer organisms
under particularconditions
Nucleic acids (DNARNA) carbohydrates
lipids amino acidspeptides and proteins
Alkaloids terpenespolyketides
polyphenols andtheir heterosidic form
Biological effects(defense signaling)
Biosyntheticpathways
Main compoundclasses
FIGURE 11 Primary versus secondary metabolisms
PS-I
PS-IIendash
hν H2O H+ and O2
ATP synthase
NADP reductase
H+ (gradient)
H+ADP + Pi ATP
NADPHH+NADP+
H+
Thylakoidcompartment
Chloroplaststroma
Thylakoidmembrane
(a) Light dependent process
(b) Light independent process
CO2
RuBP
Rubisco
Unstable β-ketoacid3-PGA 13-diPGA
ATP
ADP3-PGAL
NADPHH+
NADP+
(Calvin cycle)
trioses tetroses pentoses hexoses (eg glucose) heptoses
Chloroplaststroma
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
CH2OPO32ndash CH2OPO3
2ndash CH2OPO32ndash
CH2OPO32ndash
OOH
HO
HOO
HO
HO2CCO2H
HO
CO2PO32ndash
HO
CHOHO
endash
Cytosol
SCHEME 11 The photosynthetic machinery (PS‐I and PS‐II photosystems I and II)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 3
chloroplast mEP is a starting block in the biosynthesis of terpenes through C
5 isoprene units (isopentenyl diphosphate
(IPP) and dimethylallyl diphosphate (DmAPP)) especially those in C
10 C
20 and C
40 terpenes 3‐PGA is a precursor of
serine and other amino acids while phosphoenolpyruvate (PEP) the precursor of PA is also an intermediate toward the previously mentioned shikimic acid Lastly PA is not only a precursor of the fundamental ldquoC
2rdquo acetyl coenzyme A
(AcCoA) unit but also an intermediate toward aliphatic amino acids and mEP
AcCoA is the building block of fatty acids polyketides and mevalonic acid (mVA) a cytosolic precursor of the C
5 isoprene units for the biosynthesis of terpenes in the
C15
and C30
series (mind it is different from the mEP pathway in product and in cell location) Finally AcCoA enters the citric acid or Krebs cycle which leads to several precursors of amino acids These are oxaloacetic acid precursor of aspartic acid through transamination (thus toward lysine as a nitrogenated C
5N linear unit and methishy
onine as a methyl supplier) and 2‐oxoglutaric acid preshycursor of glutamic acid (and subsequent derivatives such as ornithine as a nitrogenated C
4N linear unit) All these
amino acids are key precursors in the biosynthesis of many alkaloids
114 Reactions Involved in the Construction of Secondary Metabolites
most reactions occurring in the living cells are performed by specialized enzymes which have been classified in an intershynational nomenclature defined by an enzyme commission (EC) number There are six classes of enzymes depending on the biochemical reaction they catalyze EC‐1 oxidoreducshytases (catalyzing oxidoreduction reactions) EC‐2 transfershyases (catalyzing the transfer of functional groups) EC‐3 hydrolases (catalyzing hydrolysis) EC‐4 lyases (breaking bonds through another process than hydrolysis or oxidation leading to a new double bond or a new cycle) EC‐5 isomershyases (catalyzing the isomerization of a molecule) and EC‐6 ligases (forming a covalent bond between two molecules) many subclasses of these enzymes have been described depending on the type of atoms and functional groups involved in the reaction and if any on the cofactor used in this reaction For example several cofactors can be used by dehydrogenases like NAD(P)NAD(P)H FADFADH
2 or
FmNFmNH2 For a description of this classification the
reader can refer to specialized Internet websites like ExplorEnz [1] What is important to realize is that most enzymes are substrate specific and have been selected during
CHOHO
CO2H
O
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
HOOH
HO
HOH2CO
OHC
CH2OPO32ndash
OHHO
CO2H
HO
OH
OH
TYR
PHE
CO2H
NH2
TRP
CH2OPO32ndash
HO
HOH2COH
MEP
C10 C20 and C40 terpenes
CO2HHO
SER
GLY CYS
CO2HOPO3
2ndash
CH2OH
OHHO2C
MVA
C15 and C30 terpenes
Polyketides
Krebscycle
(citric acid)
VAL ALA ILE LEU
CO2H
O
HO2C
CO2HO
CO2H
ASP
LYS
GLU
Glucose-6-phosphate
Fructose-6-phosphate
Erythrose-4-phosphate
3-PGAL
OP2O63ndash OP2O6
3ndash
IPP DMAPP
3-PGA PEP PA
O SCoAAcCoA
3x
Cytosol
Cytosol
Chloroplasts
Chloroplasts
Glycolysis
C2
C5
Shikimic acid Oxaloaceticacid
2-Oxoglutaricacid
MET
THR
PRO
ARGORN
Alkaloids Alkaloids Alkaloids Alkaloids
Aminoacids
Peptidesproteins
Phenolics
C1
C5N C4N
ldquoCH3rdquo
(Indole)C2NC6C3C6C2N C6C1
Pentosephosphatepathway
SCHEME 12 The building block chart involving glycolysis and the Krebs cycle
4 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
evolution to perform specific transformations making natural products with often and yet unknown functions
Secondary metabolites arise from specific biosynthetic pathways which use the previously defined building blocks The bunch of organic reactions involved in these biosynshytheses allows the construction of natural product frameshyworks which are finally diversified through ldquodecorationrdquo steps (Scheme 13) It is not the purpose of this introductive chapter to describe in detail all biosynthetic pathways and the reader can refer to excellent books and articles which have been published elsewhere [2 3]
The reactions involved in the construction of natural product skeletons will be described later for representative classes of compounds The identification of the building block footprint in the natural product skeleton will be emphasized as much as possible sometimes referring to biogenetic speculations [4] After the framework construction the decoration steps will involve as diverse reactions as aliphatic CH oxidations (eg involving a cytochrome P
450 oxygenase) occasionally triggering a
rearrangement heteroatom alkylations (eg methylation by S‐adenosylmethionine) or allylation (by DmAPP) esterifications heteroatom or C‐glycosylations (leading to heterosides) radical couplings (especially for phenols) alcohol oxidations or ketone reductions amineketone transaminations alkene dihydroxylations or epoxidations oxidative halogenations BaeyerndashVilliger oxidations and further oxygenation steps At the end of the biosynthesis such transformations may totally hide the primary building block origin of natural products
115 Secondary Metabolisms
1151 Polyketides Polyketides (or polyacetates) are issued from the oligomerization of C
2 acetate units performed
by polyketide synthases (PKS) and leading to (C2)
n linear
intermediates [5 6] If the (C2)
n intermediates arise from
successive Claisen reactions performed by ketosynthase
domains (KS in nonreducing PKS) a highly reactive poly‐ β‐ketoacyl intermediate H(CH
2CO)
nOH is formed
leading to phenolic and aromatic products through further intramolecular Claisen condensations Furthermore highly reducing PKSs are made of specialized enzymatic subunits working in line or iteratively to functionalize each C
2 linker
bond as CH(OH)CH2 (by ketoreductases (KR)) then as
HCCH (by dehydratases (DH)) and as CH2CH
2 (by enoyl
reductases (ER)) leading to a high degree of functionalization of the final product (Fig 12) By these iterative sequences highly reduced polyketides which can be either linear macrocyclized or polycyclized depending on the reactivity of the biosynthetic intermediates can be formed [7] With the same logic fatty acids are also biosynthesized by fatty acid synthases
moreover the PKS enzyme can be hybridized with nonshyribosomal peptide synthetase (NRPS) domains (see also ldquoNRPS metabolites and peptidesrdquo in the ldquoAlkaloidsrdquo secshytion) leading to the acylation of an amino acid by the (C
2)
n
acyl intermediate As previously the functionalization of the acyl chain depends on the PKS enzyme and the PKSNRPS products are also extremely diversified (eg hirsutellone B Fig 12) [8]
1152 Terpenes Terpenes are derived from the oligoshymerization of the C
5 isoprene units DmAPP and IPP Both
precursors are prompt to generate either an allylic cation (the diphosphate is a good leaving group) or a tertiary carbocation respectively which makes the IPP easy to react with DmAPP (Scheme 14) This reaction happens in the active site of a terpene synthase which activates the deparshyture of the diphosphate group from DmAPP thanks to Lewis acid activation (a metal like mg2+ mn2+ or Co2+ is present in the enzyme active site [9]) This leads to geranyl (C
10
monoterpene precursor) or farnesyl (C15
sesquiterpene precursor) diphosphate depending on the location of the enzyme (chloroplast for the mEP pathway or cytosol for the mVA pathway) Geranylgeranyl (C
20 diterpene) and
Constructionreactions
Decoration reaction(functionalization)Building
blocksNatural product
frameworksNatural products
HH
OH OAcH
HO O OH
OHO
OBz
P450
P450
P450
P450
P450 P450 Oxidativeauxiliaryenzymes
Taxadiene
OP2O63ndash
OP2O63ndash
3times
(a)
(b)
10-Deacetylbaccatin III
DMAPP
IPPand electrophilic
cyclizations
SCHEME 13 (a) From building blocks to natural products and (b) the example of 10‐deacetylbaccatin III
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 5
farnesylfarnesyl (C30
triterpene precursor) diphosphates can also be obtained by further additions of IPP leading to longer linear intermediates
The cyclization of linear precursors is achieved by speshycialized cyclases which generate a poorly functionalized natural product framework [10 11] Auxiliary enzymes such as oxygenases then increase the complexity and the diversity
of compounds by further functionalization (Scheme 13b) [12] A high degree of oxidation can be observed in comshypounds like thapsigargin paclitaxel or bilobalide (Fig 13) The biosynthesis of this last compound for example involves a high oxygenation pattern two Wagnerndashmeerwein rearrangements and several oxidative cleavages leading to the loss of five carbons The resulting natural products can
KS
S-ACP
O O
KR
S-ACP
OH O
S-MAT
O
YesNo
R
R
R
YesNoDH
S-ACP
OR
YesNoER
S-ACP
OR
YesNoTE
OH
OR
New cycle
New cycleor
release
New cycleor
release
New cycleor
release
Release
R in
crem
ente
d by
two
carb
ons
HO
O
S-ACP
O
Claisen condensation (ndashCO2)
reduction (NADPH)
dehydration (ndashH2O)
+
reduction (NADPH)
hydrolysis (H2O)
O O NH
OH
OH
H H
H H
Hirsutellone B(mixed PKSNRPS
product)
OO
O
OHOH
HO OH
Norsolorinic acid
O
O
O
O CHO
OH
HH
H
H
HH
OHHemibrevetoxin B
OOHO
OHO
HOHO OH
Erythronolide A
OH
OH
HO
Panaxytriol
From ahexanoylstartingblock
H
H
Fromtyrosine
H
1C lost fromdecarboxylation
FIGURE 12 Chemical logic of polyketide construction leading to variable functionalization of the elongated acyl chain and examples of resulting chemical diversity ACP acyl carrier protein DH dehydratase ER enoyl reductase KR ketoreductase KS ketosynthase mAT malonyl acyl transferase TE thioesterase
DMAPPOP2O6
3ndashOP2O6
3ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
Geranyl diphosphate (GPP)2 times IPP
H H
Geranylgeranyl diphosphate (GGPP)
Examplesverticillane
casbanetaxane
phorbol
Exampleslabdane
pimaranekauraneabietane
aphidicolanegibberellane
IPP
SCHEME 14 Early assembly of C5 units in terpene biosynthesis leading to diterpenes (C
20)
6 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
thus be extremely modified with structures whose biogenetic origin is far from being obvious at first sight and cannot be determined without further experiments such as isotopic labeling
1153 Flavonoids Resveratrols Gallic Acids and Further Polyphenolics We have previously discussed the polyketide origin of some phenolic compounds based on the (C
2)
n motif Other polyphenols like gallic acids are directly
derived by the aromatization of shikimic acid (C6C
1 building
block Scheme 12) [13] The C6C
3 building blocks are availshy
able from the conversion of phenylalanine and tyrosine into cinnamic and p‐coumaric acids respectively and then by further hydroxylation steps (Scheme 15) These can dimerize into lignans (eg podophyllotoxin) [14 15] through radical processes or converted to low molecular weight compounds like eugenol coumarins or vanillin [16] The coenzyme A thioesters of these C
6C
3 acids can be used as initiator units
by specialized ketosynthases for an elongation by two acetyl units leading to aromatic polyketides like styrylpyrones or diarylheptanoids (eg curcumin) [17] Important comshypounds from this metabolism are flavonoids (C
6C
3C
6) [18]
and stilbenoids (C6C
2C
6) (a decarboxylation occurs during
the aryl cyclization) [19] which are synthesized by chalcone synthase and stilbene synthase respectively Flavonoids (eg catechin) and stilbenes (eg resveratrol) are present in large amounts in fruits and vegetables and may exert their radical scavenging properties in vivo
1154 Alkaloids Alkaloids are nitrogen‐containing compounds The nitrogen(s) can be involved in an amine function conferring basicity to the natural product (like ldquoalkalirdquo) or in less or nonbasic functions such as an amide a nitrile an isonitrile or an ammonium salt (quaternary amines) For amines protonation often occurs at physiological pH and may condition their biological activity In many cases the nitrogen is biogenetically derived from an amino acid We will thus discuss alkaloids according to their amino acid origin
Alkaloids Derived from the Krebs Cycle (Lysine and Ornithine Derived) As shown previously (Scheme 12) the Krebs cycle is a crucial metabolic process which leads to α‐ketoacids (oxaloacetic and 2‐oxoglutaric acids) Their enzymatic transamination affords the two amino acidsmdashaspartic acid and glutamic acidmdashwhich are the direct biosynthetic precursors of amino acids lysine and ornithine respectively These in turn produce cadaverine a ldquoC
5Nrdquo unit
and putrescine a ldquoC4Nrdquo unit which are major components
for the biosynthesis of important alkaloids as will be discussed later (Scheme 16) Additionally ornithine is a precursor of arginine another important amino acid
ornithine‐derived alkaloids (incorporating the c4n
unit) Putrescine is derived from the decarboxylation of ornithine and is a precursor of linear polyamines like spermshyine After enzymatic methylation of one amine of putrescine
C10 C15
C15
C10
C10
C30
CO2H
Chrysanthemic acid
OH
Menthol
O
HO
H
HOGlc
MeO2CLoganin (iridoid)
H
H
H
HH
OO
OParthenolide
O
O
OHOHO
OAcHO
OnC3H7
O
O
O
nC7H15
Thapsigargin
O
O
OO
H
H
O
Cleavage
H
H
Artemisinin
C20 C40
O
O
OO
OO
OHOHH
Bilobalide(highly rearrangedand missing 5 C)
CleavageH
Highoxidativecleavages
HO OAcH
AcO O OH
OOOBz
O
Ph
BzNH
OHFrom PHE Paclitaxel
C25O
H
HO
OHCH
OH Ophiobolin A
H
HO
H
H
HHCholesterol
(missing 3 C WM)
H
H H H
H
H
Hopene
β-Carotene
C30 - 3
C15
C20 - 5WM
WM
FIGURE 13 Chemical diversity in the terpene series (Wm Wagnerndashmeerwein shifts lost carbons bold bonds are remnant of primary building blocks)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 7
in the presence of S‐adenosylmethionine transamination of the other affords γ‐(N‐methylamino)aldehyde [20] The resulting cyclic iminium is a key intermediate in the formation of many medicinally important alkaloids such as
the plant‐derived compounds cocaine atropine or the calysshytegines [21 22] Indeed this iminium is a mannich acceptor which can react with various nucleophiles the first of those being the carbanion of acetyl‐CoA Thus after a stepwise
CO2H
RCinnamic acid (R=H)p-Coumaric acid (R=OH)
RO
PHETYR
OH
[H] [O]
lignans
OH
O
O
O
O
OMeMeO
OMe
C mdash C andor C mdash Oradical couplings
After E Zisomerization
for example Podophyllotoxin
O ORO
Coumarins(eg scopoletin)
[O]
Prenylationcyclizationcleavage[O]
O OO
Furocoumarins(eg psoralen)
RO
nAcetyl-CoA
thencyclization
n = 2 styrylpyrones
O O
OMe
n = 3 avonoids(from chalcone synthase)
C6C3
H
H H
From AcCoA
O
n = 3 stilbenoids(from stilbene synthase)
HO
OH
OHFrom AcCoA From AcCoA(ndashCO2)
Resveratrol
HO
OH
OH
OH
OH
H
Catechin
MeO Yangonin
From AcCoA
SCHEME 15 The phenylpropanoid biosynthetic pathways
LYS
CO2
Cadaverine
O NH
H2O
NH
Piperideineiminium
Pelletierine
AcAcCoA
CO2
NH
O
N
O
Pseudopelletierine
NH
Ph
OH
Ph
O
Lobeline
NH
δ+
δndash
N
H
H
N
H
OH
Lupinine
N
H N
H
(+)-Sparteine
N
NH
O (ndash)-Cytisine
NH
CO2H
Pipecolic acid
N
OHHO
OH
HO
H
Castanos permine
ORN
CO2
NH2 NH2 NH2 NH2
NH2
NH2
NH2
PutrescineO
NHMe
N-Methylpyrrolinium
2 timesAcCoA
NMe
ARG
n = 1 spermidinen = 2 homospermidine
NMe
O
HygrineCO2
CO2
MeN
O
[O]
TropinoneCO2
HNHO
OHOH
OH
Calystegine B2
MeN
OAtropine
O
Ph
HO
NMe
NMe
NMe
O
Cuscohygrine
HN
HN
NH2
( )n
O
n = 2
N
HHO
Retronecine
HH
H
H
H
HO
H H
H
SCHEME 16 Lysine‐ and ornithine‐derived alkaloid biosynthetic pathways (mind the structural similarities)
8 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
elongation by two AcCoA units either decarboxylation can occur leading to the acetonylpyrrolidine hygrine or a secshyond mannich reaction by the intramolecular attack of the acetoacetate anion onto an oxidation‐derived pyrrolinium leading to the tropane skeleton (tropinone) The acetoacetyl‐CoA intermediate can also react intermolecularly with another pyrrolinium cation leading to cuscohygrine after decarboxylation Finally the pyrrolizidine alkaloids [23] are derived from homospermidine which when submitted to terminal oxidative deamination leads to the bicyclic skelshyeton of retronecine and further Senecio alkaloids We can mention herein that ornithine is a biosynthetic precursor of arginine bearing a guanidine function which is an intermediate toward the toxic compounds tetrodotoxin and saxitoxin (not shown)
lysine‐derived alkaloids (incorporating the c5n
unit) From lysine to piperidine alkaloids the biosynthetic steps parallel the one previously described from ornithine Indeed the oxidative deamination of cadaverine affords a δ‐amino aldehyde which cyclizes through imine formation into piperideine Protonation results in a mannich acceptor which is able to react with various nucleophiles such as β‐ketothioester anions The first product of these reactions is pelletierine which can further react through an intramolecular mannich
reaction leading to pseudopelletierine Quinolizidines [24] can also be formed first from the mannich reaction of the piperideine acceptor with the corresponding enamine nucleoshyphile and then after additional transformation steps leading for example to lupinine sparteine or cytisine
Indolizidine alkaloids [15] such as castanospermine and swainsonine are formed from pipecolic acid an amino acid derived from lysine which can be elongated by malonyl‐CoA followed by ring closure When protonated these alkaloids are oxonium mimics strongly inhibiting glycosidases
Tyrosine‐ and Phenylalanine‐Derived Alkaloids Tyrosine and phenylalanine amino acids are bearing the phenylshyethylamine moiety of many medicinally relevant alkaloids Further hydroxylations on the aromatic carbocycle or on the aliphatic part can be observed methylations can occur on phenolic oxygens and on the amine leading to catecholamines (adrenaline noradrenaline dopamine) Arylethylamines are also usual to react with endogenous aldehydes through PictetndashSpengler reactions [25] leading to important biosynthetic intermediates (Scheme 17) like
bull Reticuline from the reaction with 4‐hydroxyphenylacetshyaldehyde toward benzyltetrahydroisoquinoline alkaloids
NH2
Phenylethylamines
RONH TetrahydroisoquinolinesRO
RʹCHO
Rʹ
NH
(CH2)n
MeO
HO
Benzyltetrahydroisoquinoline(n = 1) for example reticuline
orPhenethyltetrahydroisoquinoline
(n = 2) eg autumnaline
IntermolecularCmdashO phenol
couplings
Curarealkaloids
IntramolecularC mdash C phenol
couplings
ONMe
HHO
H
HO
Morphine
NMe
MeO
HO
MeOOH
Isoboldine
O
O
OMe
NO2
CO2H
Aristolochic acid
OMe
O
NHAcMeO
MeOMeO
Colchicine
N
O
O
OMe
OMeBerberine
IntramolecularCmdash C coupling
and furtheroxidative events
Rʹ derivedfrom PHE
Pictet-Spenglerreaction
TYR
HO
HO CHO
HNHO
HO
OH
OMeO
OH
NMe
[O]
GalantamineNorbelladine
Rʹ derived fromsecologanin
NAc
HO
HO
O
CO2MeH
H
H
OHIpecosideaglycone
Ipecac alkaloids
1ClostCmdash C cleavage
and ring extension
H
ROH
1C lost
Cleavage
SCHEME 17 Tyrosine‐derived alkaloid biosynthetic pathways (double head arrows figure bond cleavages during biosynthetic processes)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 9
morphine berberine tubocurarine isoboldine or the highly modified aristolochic acid [26 27]
bull Automnaline from the reaction with 3‐(4‐hydroxyphenyl)propanal toward phenylethyltetrahydroisoquinoline alkaloids colchicine cephalotaxine or schelhammerishycine [28 29]
bull Ipecoside from the reaction of dopamine with secoloshyganin toward terpene tetrahydroisoquinoline alkaloids ipecoside or emetine
Lastly norbelladine (top of Scheme 17) is issued from the reductive amination of 34‐dihydroxybenzaldehyde (derived from phenylalanine) with tyramine (derived from tyrosine) and constitutes a biosynthetic node leading to Amaryllidaceae alkaloids such as galantamine crinine or lycorine depending on the topology of phenolic couplings In all these biosynthetic routes radical phenolic couplings are key reactions for CC and CO bond formations and rearrangements [30 31]
Tryptophan‐Derived Indole and Indole Monoterpene Alkaloids As for alkaloids derived from tyrosine and phenylalanine those derived from tryptophan are formed after decarboxylation of the amino acid (into tryptamine) and possible hydroxylation of the aromatic carbocycle (eg serotonin) and N‐methylation (eg psilocin) As previously tryptamine can also react through PictetndashSpengler reactions to form tetrahydro‐β‐carbolines which can be aromatized for example into harmine (Scheme 18) [16]
When the aldehyde partner of the PictetndashSpengler reacshytion with tryptamine is the terpene secologanin strictosidine is formed as an entry toward the vast monoterpene indole alkaloids [32 33] Hydrolysis of the glucosidic part releases the strictosidine aglycone bearing an aldehyde while iminshyium formation and further cyclization and reduction can lead to ajmalicine (from oxocyclization) or yohimbine (from carshybocyclization) These alkaloids are referred to as from the Corynanthe type with the monoterpene carbon skeleton unmodified Although it misses one carbon and has a very
RO
Carbonylcompounds
TRPNH
NH2
Indolethylamines(eg serotonine)
RONH
NH
Rʹ NH
N
MeOPictet-Spenglerreaction for example Harmineβ-Carbolines
NH
NHH
O
MeO2C
MeO2C
OH
H
H
Rʹ derived from secologanin
Strictosidine aglycone
NH
N
OH
H
H
Ajmalicine
N
N
O
O
H
H
H
Strychnine (C lost)Corynanthe type
Aspidosperma type Iboga type
N
N
OH
OMe
Quinine (C lost)
N
NH CO2Me
Catharanthine
NH
N
CO2MeH
OAc
OH
Vindoline
Complextransformations
NH
NMe
HN
MeN
H
H
Chimonanthine
Cyclizationdimerization
Prenylationcyclization
NH
NMeHO2C
H
Lysergic acid
H
H
Ergot alkaloidsPyrroloindoles Indole monoterpene
1C lost
1C lostFrom AcCoA
SCHEME 18 Tryptophan‐derived alkaloid biosynthetic pathways (gray parts monoterpenic units)
10 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
different structure strychnine is related to the Corynanthe alkaloids incorporating two carbons from acetyl‐CoA Highly modified monoterpene skeletons are derived from the Corynanthe core through CC bond breaking and reorshyganization leading to Iboga‐type (eg catharanthine) and Aspidosperma‐type (eg vindoline) alkaloids The antishycancer drug vinblastine is a heterodimer resulting from the nucleophilic attack of vindoline on a mannich acceptor resulting from catharanthine found in madagascar perishywinkle (Catharanthus roseus) The heteroaromatic comshypounds ellipticine camptothecin and quinine are also derived from a Corynanthe‐type precursor although in this case the biosynthetic relationship may not be obvious due to deep modifications of the skeleton
Finally two important classes of compounds have to be mentioned since they have inspired many synthetic chemists The pyrroloindole alkaloids result from the cyclization of tryptamine as found in physostigmine (formed by a cationic mechanism after methylation in position 3 of the indole not shown) or in chimonanthine (presumably formed by a radical coupling mechanism Scheme 18) The ergot alkaloids are derived from the 33‐dimethylallylation on position 4 of the indole in trypshytophan whose further cyclization and oxidation processes afford the natural products (eg lysergic acid Scheme 18 and ergotamine) which have had important medical applications [34]
NRPS Metabolites and Peptides NRPS enzymes assemble amino acids including nonproteinogenic ones into oligopeptides The enzymes contain several modules and especially an adenylation domain (A) which specifically selects and activates the amino acid to be transferred as a thioester on the nearby peptidyl carrier protein (PCP) [2] A condensation module (C) then catalyzes the formation of the peptide bonds between the newly introduced amino acyl‐PCP (bearing a free amine) and the elongated peptidyl‐PCP thioester At the end of the elongation a cyclization can occur into cyclopeptides but the peptide can also be
transferred to auxiliary enzymes like methyltransferases glycosyltransferases or oxidases (vancomycins are typical products of such functionalizations) [35 36] The formation of heterocycles is also frequently encountered in this metabolism as in penicillins that are derived from the tripeptide α‐aminoadipoyl‐cysteinyl‐valine or telomestatin (Fig 14) [2]
Other Alkaloid Origins There are many other nitrogen sources involved in alkaloid biosyntheses for example nicotinic acid (originated from aspartic acid and intershymediate in nicotine and anabasine biosyntheses) and anthranilic acid (originated from tryptophan and intermedishyate toward acridines or aurachins) The amination reaction (eg through transamination of carbonyl compounds) is also a way to introduce nitrogens in natural products for example from fatty acids steroids (toward Solanum alkaloids or cyclopamines) or other terpenoids (aconitine and atisine have diterpene skeletons while Daphniphyllum alkaloids are triterpene derivatives) Finally nucleic acids can also be precursors of alkaloids like the well‐known caffeine
12 FROM BIOSYNTHESIS TO TOTAL SYNTHESIS STRATEGIES TOWARD THE NATURAL PRODUCT CHEMICAL SPACE
121 The Chemical Space of Natural Products
Natural products occupy an important place in human com munities as demonstrated by their vast use from ancient times to nowadays like dyes fibers oils pershyfumes agrochemicals or drugs Broadly both primary and secondary metabolites could be classified as natural products while the latter as discussed previously are usushyally regarded as the ldquonatural productsrdquo owing to their comshyplexity and diversity arising from a variety of biosynthetic pathways The structural chemical diversity found among
N
S
CO2HO
HNPhH2C
O
Penicillin G
OH
HN
HN
O
O
ONH2
O
OHO
NHMe
OCl
O
NH
NH
NH
O
ClHO
O
HN
H
H
HOOH
OHHO2C
Vancomycin aglycone
ON
NO
O
NN
O
O
N
N O
Me
S
N N
OMeH
Telomestatin
L-AAA-L-CYS-D-VAL
Enzymes
FIGURE 14 Structural diversity of nonribosomal peptide compounds (AAA α‐aminoadipic acid)
FROm BIOSyNTHESIS TO TOTAL SyNTHESIS STRATEGIES TOWARD THE NATuRAL PRODuCT CHEmICAL SPACE 11
all living organisms defining the chemical space of natural products [37] is the consequence of their evolution occurshyring as an adaptation of organisms to their environment It is commonly believed that secondary metabolites are proshyduced as messengers by living organisms or as weapons against enemies and thus they should have certain biological activities in a medicinal point of view [38] Indeed natural products are regarded as one of the main sources of medicines (Fig 15) From the traditional medicinal extracts to every single bioactive molecule the methods of extraction purification identification and biological investigation of natural products have been well established Their complex structures and interesting properties have attracted synthetic chemists to accomplish their total syntheses and that of medicinally relevant anashylogs sometimes in the industrial context [39] Thus tarshygeting the chemical space of natural products has never been more relevant than today Although the discovery of natural products demands time and labor‐consuming manipulations it is worth to notice that the knowledge on this chemical space is still continually growing while biological advances allow for discovering and undershystanding potential targets However increasing the chemical space of human‐made compounds based on natural products should benefit from transdisciplinary colshylaborations such as the use of coupled biosynthetic and chemical synthetic methods to design original ldquounnatural natural productsrdquo [40]
122 The Biosynthetic Pathways as an Inspiration for Synthetic Challenges
1221 Precursor‐Directed Biosyntheses and Mutashysynthetic Strategies to Increase the Chemical Space of Natural Products As the genetics and biochemistry of natural product biosynthesis are better understood novel biosynthetic techniques have been developed to study and generate new diversity in natural product analogs (Scheme 19) Precursor‐directed biosynthesis (PDB) is considered as the earliest example of combining chemical and biological methods for the generation of complex natural product analogs [41 42] This approach compared with the biosynthetic pathway of wild‐type metabolites (Scheme 19a) involves the feeding of analogs of the natural biosynthetic building blocks to the living organisms (Scheme 19b) usually bacteria or fungi which incorporate the modified precursors into the biosynthesized compound mutasynthesis also termed as mutational biosynthesis (mBS) involves the inactivation of a key step of the bioshysynthesis in a mutant microorganism (Scheme 19c) which can then be fed by various modified or advanced building blocks (mutasynthons Scheme 19d) [43] These mutasshyynthons could not be incorporated by the wild type due to specificities of the enzymatic machinery Build up on PDB mBS eliminates the natural biosynthetic intermediate thus generating a less complex mixture of metabolites and making the purification or yield of target products better
O NH
OHO
O
OO
O
O
OH
HOO
O O
OH
HOO
NH ONH2
O
ONH
NH
O
OCl Cl
O
O
OH
HN
HN
HN
HN
OO
HO
HN
OH
OHOH
O
O
HO
HO
OHO
O
OHH2N
O
OH3C
H
O
H
CH3
CH3
H
O O
NO
H
O
HO
O
NO
O
NH
HN
OHO
HONH
ON
H
HO
O
H2N
O OH
ONH
OH
ON OHO
OH
HO OS OH
OOOH
OHO
O
ON
OO
O
HO
O
O OH
OO
Micafunginantifungal
Rapamycinimmunosuppressive
Galantamineanti-Alzheimer
Taxolanticancer
Artemisininantimalarial
Vancomycinantibiotic
FIGURE 15 Some famous natural products currently used as drugs
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product
LIST OF CONTRIBUTORS xiii
PREFACE xv
1 From Biosyntheses to Total Syntheses An Introduction 1Bastien Nay and Xu‐Wen Li
11 From Primary to Secondary Metabolism the Key Building Blocks 1111 Definitions 1112 Energy Supply and Carbon Storing at the Early Stage
of Metabolisms 1113 Glucose as a Starting Material toward Key Building Blocks
of the Secondary Metabolism 1114 Reactions Involved in the Construction of Secondary Metabolites 3115 Secondary Metabolisms 4
12 From Biosynthesis to total Synthesis Strategies toward the Natural Product Chemical Space 10121 the Chemical Space of Natural Products 10122 the Biosynthetic Pathways as an Inspiration
for Synthetic Challenges 11123 the Science of total Synthesis 14124 Conclusion a Journey in the Future of total Synthesis 16
References 16
SECTION I ACETATE BIOSYNTHETIC PATHWAY 19
2 Polyketides 21Franccediloise Schaefers Tobias A M Gulder Cyril Bressy Michael Smietana Erica Benedetti Stellios Arseniyadis Markus Kalesse and Martin Cordes
21 Polyketide Biosynthesis 21211 Introduction 21212 assembly of acetateMalonate‐Derived Metabolites 23213 Classification of Polyketide Biosynthetic Machineries 23214 Conclusion 39
References 40
CONTENTS
viii CoNtENtS
22 Synthesis of Polyketides 44221 asymmetric alkylation Reactions 44222 applications of asymmetric alkylation Reactions in total Synthesis
of Polyketides and Macrolides 60References 8323 Synthesis of Polyketides‐Focus on Macrolides 87
231 Introduction 87232 Stereoselective Synthesis of 13‐Diols asymmetric aldol Reactions 88233 Stereoselective Synthesis of 13‐Diols asymmetric Reductions 106234 application of Stereoselective Synthesis of 13‐Diols in
the total Synthesis of Macrolides 117235 Conclusion 126
References 126
3 Fatty Acids and Their Derivatives 130Anders Vik and Trond Vidar Hansen
31 Introduction 13032 Biosynthesis 130
321 Fatty acids and Lipids 130322 Polyunsaturated Fatty acids 134323 Mediated oxidations of ω‐3 and ω‐6 Polyunsaturated
Fatty acids 13533 Synthesis of ω‐3 and ω‐6 all‐Z Polyunsaturated Fatty acids 140
331 Synthesis of Polyunsaturated Fatty acids by the Wittig Reaction or by the Polyyne Semihydrogenation 140
332 Synthesis of Polyunsaturated Fatty acids via Cross Coupling Reactions 143
34 applications in total Synthesis of Polyunsaturated Fatty acids 145341 Palladium‐Catalyzed Cross Coupling Reactions 145342 Biomimetic transformations of Polyunsaturated Fatty acids 149343 Landmark total Syntheses 153344 Synthesis of Leukotriene B
5 158
35 Conclusion 160acknowledgments 160References 160
4 Polyethers 162Youwei Xie and Paul E Floreancig
41 Introduction 16242 Biosynthesis 162
421 Ionophore antibiotics 162422 Marine Ladder toxins 165423 annonaceous acetogenins and terpene Polyethers 165
43 Epoxide Reactivity and Stereoselective Synthesis 166431 Regiocontrol in Epoxide‐opening Reactions 166432 Stereoselective Epoxide Synthesis 172
44 applications to total Synthesis 176441 acid‐Mediated transformations 176442 Cascades via Epoxonium Ion Formation 179443 Cyclizations under Basic Conditions 181444 Cyclization in Water 182
45 Conclusions 183References 184
CoNtENtS ix
SECTION II MEVALONATE BIOSYNTHETIC PATHWAY 187
5 From Acetate to Mevalonate and Deoxyxylulose Phosphate Biosynthetic Pathways An Introduction to Terpenoids 189Alexandros L Zografos and Elissavet E Anagnostaki
51 Introduction 18952 Mevalonic acid Pathway 19153 Mevalonate‐Independent Pathway 19254 Conclusion 194References 194
6 Monoterpenes and Iridoids 196Mario Waser and Uwe Rinner
61 Introduction 19662 Biosynthesis 196
621 acyclic Monoterpenes 197622 Cyclic Monoterpenes 197623 Iridoids 200624 Irregular Monoterpenes 202
63 asymmetric organocatalysis 203631 Introduction and Historical Background 204632 Enamine Iminium and Singly occupied Molecular
orbital activation 207633 Chiral (Broslashnsted) acids and H‐Bonding Donors 213634 Chiral BroslashnstedLewis Bases and Nucleophilic Catalysis 218635 asymmetric Phase‐transfer Catalysis 220
64 organocatalysis in the total Synthesis of Iridoids and Monoterpenoid Indole alkaloids 225641 (+)‐Geniposide and 7‐Deoxyloganin 226642 (ndash)‐Brasoside and (ndash)‐Littoralisone 227643 (+)‐Mitsugashiwalactone 229644 alstoscholarine 229645 (+)‐aspidospermidine and (+)‐Vincadifformine 230646 (+)‐Yohimbine 230
65 Conclusion 231References 231
7 Sesquiterpenes 236Alexandros L Zografos and Elissavet E Anagnostaki
71 Biosynthesis 23672 Cycloisomerization Reactions in organic Synthesis 244
721 Enyne Cycloisomerization 245722 Diene Cycloisomerization 257
73 application of Cycloisomerizations in the total Synthesis of Sesquiterpenoids 266731 Picrotoxane Sesquiterpenes 266732 aromadendrane Sesquiterpenes Epiglobulol 267733 CubebolndashCubebenes Sesquiterpenes 267734 Ventricos‐7(13)‐ene 270735 Englerins 271736 Echinopines 271737 Cyperolone 273
x CoNtENtS
738 Diverse Sesquiterpenoids 27674 Conclusion 276References 276
8 Diterpenes 279Louis Barriault
81 Introduction 27982 Biosynthesis of Diterpenes Based on Cationic Cyclizations
12‐Shifts and transannular Processes 27983 Pericyclic Reactions and their application in the Synthesis
of Selected Diterpenoids 284831 Dielsndashalder Reaction and Its application in the total
Synthesis of Diterpenes 284832 Cascade Pericyclic Reactions and their application in the total
Synthesis of Diterpenes 29184 Conclusion 293
References 294
9 Higher Terpenes and Steroids 296Kazuaki Ishihara
91 Introduction 29692 Biosynthesis 29693 Cascade Polyene Cyclizations 303
931 Diastereoselective Polyene Cyclizations 303932 ldquoChiral proton (H+)rdquo‐Induced Polyene Cyclizations 304933 ldquoChiral Metal Ionrdquo‐Induced Polyene Cyclizations 308934 ldquoChiral Halonium Ion (X+)rdquo‐Induced Polyene Cyclizations 313935 ldquoChiral Carbocationrdquo‐Induced Polyene Cyclizations 319936 Stereoselective Cyclizations of Homo(polyprenyl)arene
analogs 31994 Biomimetic total Synthesis of terpenes and Steroids through
Polyene Cyclization 31995 Conclusion 328
References 328
SECTION III SHIKIMIC ACID BIOSYNTHETIC PATHWAY 331
10 Lignans Lignins and Resveratrols 333Yu Peng
101 Biosynthesis 3331011 Primary Metabolism of Shikimic acid and aromatic
amino acids 3331012 Lignans and Lignin 335
102 auxiliary‐assisted C(sp3)ndashH arylation Reactions in organic Synthesis 336
103 FriedelndashCrafts Reactions in organic Synthesis 344104 total Synthesis of Lignans by C(sp3)H arylation Reactions 353105 total Synthesis of Lignans and Polymeric Resveratrol by
FriedelndashCrafts Reactions 357106 Conclusion 375References 375
CoNtENtS xi
SECTION IV MIXED BIOSYNTHETIC PATHWAYSndash THE STORY OF ALKALOIDS 381
11 Ornithine and Lysine Alkaloids 383Sebastian Brauch Wouter S Veldmate and Floris P J T Rutjes
111 Biosynthesis of l‐ornithine and l‐Lysine alkaloids 3831111 Biosynthetic Formation of alkaloids
Derived from l‐ornithine 3831112 Biosynthetic Formation of alkaloids
Derived from l‐Lysine 388112 the asymmetric Mannich Reaction in organic Synthesis 392
1121 Chiral amines as Catalysts in asymmetric Mannich Reactions 3941122 Chiral Broslashnsted Bases as Catalysts in asymmetric
Mannich Reactions 3981123 Chiral Broslashnsted acids as Catalysts in asymmetric
Mannich Reactions 4041124 organometallic Catalysts in asymmetric Mannich Reactions 4081125 Biocatalytic asymmetric Mannich Reactions 413
113 Mannich and Related Reactions in the total Synthesis of l‐Lysine‐ and l‐ornithine‐Derived alkaloids 414
114 Conclusion 426References 427
12 Tyrosine Alkaloids 431Uwe Rinner and Mario Waser
121 Introduction 431122 Biosynthesis of tyrosine‐Derived alkaloids 431
1221 Phenylethylamines 4311222 Simple tetrahydroisoquinoline alkaloids 4331223 Modified Benzyltetrahydroisoquinoline alkaloids 4331224 Phenethylisoquinoline alkaloids 4361225 amaryllidaceae alkaloids 4381226 Biosynthetic overview of tyrosine‐Derived alkaloids 442
123 arylndasharyl Coupling Reactions 4421231 Copper‐Mediated arylndasharyl Bond Forming Reactions 4431232 Nickel‐Mediated arylndasharyl Bond Forming Reactions 4461233 Palladium‐Mediated arylndasharyl Bond Forming Reactions 4471234 transition Metal‐Catalyzed Couplings of Nonactivated
aryl Compounds 450124 Synthesis of tyrosine‐Derived alkaloids 456
1241 Synthesis of Modified Benzyltetrahydroisoquinoline alkaloids 4561242 Synthesis of Phenethylisoquinoline alkaloids 4601243 Synthesis of amaryllidaceae alkaloids 462
125 Conclusion 468References 469
13 Histidine and Histidine‐Like Alkaloids 473Ian S Young
131 Introduction 473132 Biosynthesis 473133 atom Economy and Protecting‐Group‐Free Chemistry 480
xii CoNtENtS
134 Challenging the Boundaries of Synthesis PIas 488135 Conclusion 497References 499
14 Anthranilic AcidndashTryptophan Alkaloids 502Zhen‐Yu Tang
141 Biosynthesis 502142 Divergent SynthesisndashCollective total Synthesis 508143 Collective total Synthesis of tryptophan‐Derived alkaloids 510
1431 Monoterpene Indole alkaloids 5101432 Bisindole alkaloids 512
References 517
15 Future Directions of Modern Organic Synthesis 519Jakob Pletz and Rolf Breinbauer
151 Introduction 519152 Enzymes in organic Synthesis Merging total
Synthesis with Biosynthesis 520153 Engineered Biosynthesis 526154 Diversity‐oriented Synthesis Biology‐oriented Synthesis
and Diverted total Synthesis 5331541 Diversity‐oriented Synthesis 5351542 Biology‐oriented Synthesis 5361543 Diverted total Synthesis 539
155 Conclusion 541References 545
INDEX 548
Elissavet E Anagnostaki Department of Chemistry Laboratory of Organic Chemistry Aristotle University of Thessaloniki Thessaloniki Greece and Research and Development Department Pharmathen SA Thessaloniki Greece
Stellios Arseniyadis School of Biological and Chemical Sciences Queen Mary University of London London United Kingdom
Louis Barriault Department of Chemistry University of Ottawa Ottawa Ontario Canada
Erica Benedetti Laboratoire de Chimie et Biochimie et Pharmacologiques et Toxicologiques CNRS-Universiteacute Paris Descartes Faculteacute des Sciences Fondamentales et Biomeacutedicales Paris France
Sebastian Brauch Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
Rolf Breinbauer Institute of Organic Chemistry Technische Universitaumlt Graz Graz Austria
Cyril Bressy Aix Marseille Universiteacute Centrale Marseille CNRS Marseille France
Martin Cordes Institute for Organic Chemistry and Center of Biomolecular Drug Research (BMWZ) Leibniz Universitaumlt Hannover Hannover Germany and Helmholtz Center for Infection Research (HZI) Hannover Germany
Paul E Floreancig Department of Chemistry Chevron Science Center University of Pittsburgh Pittsburgh PA USA
Tobias A M Gulder Department of Chemistry and Center for Integrated Protein Science Munich (CIPSM) Biosystems Chemistry Technische Universitaumlt Muumlnchen Munich Germany
Trond Vidar Hansen School of Pharmacy University of Oslo Oslo Norway
Kazuaki Ishihara Department of Biotechnology Graduate School of Engineering Nagoya University Nagoya Japan
Markus Kalesse Institute for Organic Chemistry and Center of Biomolecular Drug Research (BMWZ) Leibniz Universitaumlt Hannover Hannover Germany and Helmholtz Center for Infection Research (HZI) Hannover Germany
Xu‐Wen Li Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
Bastien Nay Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France
Yu Peng State Key Laboratory of Applied Organic Chemistry Lanzhou University Lanzhou China
Jakob Pletz Institute of Organic Chemistry Technische Universitaumlt Graz Graz Austria
Uwe Rinner Institute of Organic Chemistry Johannes Kepler University Linz Linz Austria and Department of Chemistry College of Science Sultan Qaboos University Muscat Oman
Floris P J T Rutjes Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
LIST oF CoNTRIBUToRS
xiv LIST OF CONTRIBUTORS
Franccediloise Schaefers Department of Chemistry and Center for Integrated Protein Science Munich (CIPSM) Biosystems Chemistry Technische Universitaumlt Muumlnchen Munich Germany
Michael Smietana Institut des Biomoleacutecules Max Mousseron CNRS Universiteacute de Montpellier ENSCM France
Zhen‐Yu Tang Department of Pharmaceutical Engineering College of Chemistry and Chemical Engineering Central South University Changsha China
Wouter S Veldmate Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
Anders Vik School of Pharmacy University of Oslo Oslo Norway
Mario Waser Institute of Organic Chemistry Johannes Kepler University Linz Linz Austria
Youwei Xie Max‐Planck‐Institut fuumlr Kohlenforschung Muumllheim Germany
Ian S Young Bristol‐Myers Squibb Company Chemical Development New Brunswick NJ USA
Alexandros L Zografos Department of Chemistry Laboratory of Organic Chemistry Aristotle University of Thessaloniki Thessaloniki Greece
There is pleasure in the pathless woodsthere is rapture in the lonely shore
there is society where none intrudesby the deep sea and music in its roar
I love not Man the less but Nature moreLord Byron
Preface
The first time I came across with the idea of editing a book that merges selected chapters of biosynthesis and total synthesis was when I was teaching postgraduate courses of natural product synthesis at Aristotle University of Thessaloniki This period I realized that the best way to teach youngsters synthesis was to start from the very origin of inspiration nature and its tools biosynthesis
Over the last decades biosynthesis is filling our gaps of understanding the complex mechanisms of nature and pro-vides useful sources of inspiration not only in the way natural products can be synthesized but also by directing synthetic chemists in developing atom‐economical efficient synthetic methods Several are the examples that mimic biosynthetic guidelines from modern iterative alkylations and aldol reactions to CH oxidations that compile nowadays the modern toolbox of organic synthesis
The handed book is constructed in the logic of presenting the parallel development of biosynthesis and organic meth-odology and how these can be applied in efficient syntheses of natural products The book is divided into four sections each representing the four major biosynthetic pathways of natural products namely acetate mevalonate shikimate biosynthetic pathways and the mixed biosynthetic pathways
of alkaloids These sections are divided into chapters that represent selected classes of natural products for example lipids sesquiterpenoids lignans etc Each of these chapters is further divided into three distinct subchapters (a) biosyn-thesis (b) methodological section and (c) application of the described methodology in the total synthesis of the described family of natural products By this way the readers can be focused in the direct comparison between biosyn-theses and the developed methodologies to construct the crucial for each class of natural product carbon bonds Although the book as it develops is focused on presenting the power of biosynthesis and how this power can be applied in providing inspiration for the efficient synthesis of natural products it was not the authors will to present only biomi-metic total syntheses but rather to exploit the modern synthetic methodologies and recognize their disabilities for further improvement
Of course this book will not have been realized without the excellent work of renowned scientists worldwide working either in the field of biosynthesis or total synthesis who collected the existing knowledge on biosynthesis analyzed the existing modern methodologies and presented a bouquet of selected total syntheses Throughout our
xvi PrEfACE
endeavor to complete this book I learned many things from their expertise but I also realized that only with tight collab-orations you can build long‐lasting friendships I would like to thank them all once again for their trust and effort to complete this book We all hope that the current work will contribute to a better understanding of the current status of
organic chemistry and to the discovery of novel strategies and tactics for the synthesis of natural products
Alexandros L ZografosSeptember 2015
Thessaloniki Greece
From Biosynthesis to Total Synthesis Strategies and Tactics for Natural Products First Edition Edited by Alexandros L Zografoscopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
11 FROM PRIMARY TO SECONDARY METABOLISM THE KEY BUILDING BLOCKS
111 Definitions
The primary and secondary metabolisms are traditionally distinguished by their distribution and utility in the living organism network Primary metabolites include carbohyshydrates lipids nucleic acids and proteins (or their amino acid constituents) and are shared by all living organisms on Earth They are transformed by common pathways which are studied by biochemistry (Fig 11) Secondary metabo-lites are structurally diverse compounds usually produced by a limited number of organisms which synthesize them for a special purpose like defense or signaling through specific biosynthetic pathways They are studied by natural product chemistry This distinction is not always so obvious and some compounds can be studied in the context of both primary and secondary metabolisms This is especially true nowadays with the use of genetic and biomolecular tools which tend to make natural product sciences more and more integrative However an important point to remember is that the primary metabolism furnishes key building blocks to the secondary metabolism It would be difficult to describe in detail the full biosynthetic pathshyways in this section We tried to organize the discussion as a vade mecum synthetically gathering information from extremely useful sources which will be cited at the end of this chapter
112 Energy Supply and Carbon Storing at the Early Stage of Metabolisms
The sunlight is essential to life except in some part of the deep oceans It provides energy for plant photosynthesis that splits molecules of water into protons and electrons and releases O
2 (Scheme 11) A proton gradient inside the plant
chloroplasts then drags a transmembrane ATP synthase comshyplex that produces adenosine triphosphate (ATP) while elecshytrons released from water are transferred to the coenzyme reducer nicotinamide adenine dinucleotide phosphate hydride (NADPH) A major function of chloroplasts is to fix CO
2 as a combination to ribulose‐15‐bisphosphate (RuBP)
performed by RuBP carboxylase (rubisco) forming an instable ldquoC
6rdquo β‐ketoacid This is cleaved into two molecules
of 3‐phosphoglycerate (3‐PGA) which is then reduced into 3‐phosphoglyceraldehyde (3‐PGAL a ldquoC
3rdquo triose phosshy
phate) during the Calvin cycle This is one of the major metabolites in the biosynthesis of carbohydrates like glucose and a biochemical mean for storing and retaining carbon atoms in the living cells
113 Glucose as a Starting Material Toward Key Building Blocks of the Secondary Metabolism
Glucose‐6‐phosphate arises from the phosphorylation of glucose It is the starting material of glycolysis an important process of the primary metabolism which consists in eight enzymatic reactions leading to pyruvic acid (PA)
FROM BIOSYNTHESES TO TOTAL SYNTHESES AN INTRODUCTION
Bastien Nay1 and Xu‐Wen Li2
1
1 Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France2 Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
2 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
(Scheme 12) Important intermediates for the secondary metabolism are produced during glycolysis Glucose glucose‐6‐phosphate and fructose‐6‐phosphate can be converted to other hexoses and pentoses that can be oligoshymerized and enter in the composition of heterosides Additionally fructose‐6‐phosphate connects the pentose
phosphate pathway leading to erythrose‐4‐phosphate toward shikimic acid which is a key metabolite in the biosynthesis of aromatic amino acids (phenylalanine tyrosine or C
6C
3
units) and C6C
1 phenolic compounds The next important
intermediate in glycolysis is 3‐PGAL which can be redishyrected toward methylerythritol‐4‐phosphate (mEP) in the
Primary metabolism Secondary metabolism
The field ofbiochemistry
The field ofnatural product
chemistry
Essential toliving organisms
Essential to theproducer organisms
under particularconditions
Nucleic acids (DNARNA) carbohydrates
lipids amino acidspeptides and proteins
Alkaloids terpenespolyketides
polyphenols andtheir heterosidic form
Biological effects(defense signaling)
Biosyntheticpathways
Main compoundclasses
FIGURE 11 Primary versus secondary metabolisms
PS-I
PS-IIendash
hν H2O H+ and O2
ATP synthase
NADP reductase
H+ (gradient)
H+ADP + Pi ATP
NADPHH+NADP+
H+
Thylakoidcompartment
Chloroplaststroma
Thylakoidmembrane
(a) Light dependent process
(b) Light independent process
CO2
RuBP
Rubisco
Unstable β-ketoacid3-PGA 13-diPGA
ATP
ADP3-PGAL
NADPHH+
NADP+
(Calvin cycle)
trioses tetroses pentoses hexoses (eg glucose) heptoses
Chloroplaststroma
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
CH2OPO32ndash CH2OPO3
2ndash CH2OPO32ndash
CH2OPO32ndash
OOH
HO
HOO
HO
HO2CCO2H
HO
CO2PO32ndash
HO
CHOHO
endash
Cytosol
SCHEME 11 The photosynthetic machinery (PS‐I and PS‐II photosystems I and II)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 3
chloroplast mEP is a starting block in the biosynthesis of terpenes through C
5 isoprene units (isopentenyl diphosphate
(IPP) and dimethylallyl diphosphate (DmAPP)) especially those in C
10 C
20 and C
40 terpenes 3‐PGA is a precursor of
serine and other amino acids while phosphoenolpyruvate (PEP) the precursor of PA is also an intermediate toward the previously mentioned shikimic acid Lastly PA is not only a precursor of the fundamental ldquoC
2rdquo acetyl coenzyme A
(AcCoA) unit but also an intermediate toward aliphatic amino acids and mEP
AcCoA is the building block of fatty acids polyketides and mevalonic acid (mVA) a cytosolic precursor of the C
5 isoprene units for the biosynthesis of terpenes in the
C15
and C30
series (mind it is different from the mEP pathway in product and in cell location) Finally AcCoA enters the citric acid or Krebs cycle which leads to several precursors of amino acids These are oxaloacetic acid precursor of aspartic acid through transamination (thus toward lysine as a nitrogenated C
5N linear unit and methishy
onine as a methyl supplier) and 2‐oxoglutaric acid preshycursor of glutamic acid (and subsequent derivatives such as ornithine as a nitrogenated C
4N linear unit) All these
amino acids are key precursors in the biosynthesis of many alkaloids
114 Reactions Involved in the Construction of Secondary Metabolites
most reactions occurring in the living cells are performed by specialized enzymes which have been classified in an intershynational nomenclature defined by an enzyme commission (EC) number There are six classes of enzymes depending on the biochemical reaction they catalyze EC‐1 oxidoreducshytases (catalyzing oxidoreduction reactions) EC‐2 transfershyases (catalyzing the transfer of functional groups) EC‐3 hydrolases (catalyzing hydrolysis) EC‐4 lyases (breaking bonds through another process than hydrolysis or oxidation leading to a new double bond or a new cycle) EC‐5 isomershyases (catalyzing the isomerization of a molecule) and EC‐6 ligases (forming a covalent bond between two molecules) many subclasses of these enzymes have been described depending on the type of atoms and functional groups involved in the reaction and if any on the cofactor used in this reaction For example several cofactors can be used by dehydrogenases like NAD(P)NAD(P)H FADFADH
2 or
FmNFmNH2 For a description of this classification the
reader can refer to specialized Internet websites like ExplorEnz [1] What is important to realize is that most enzymes are substrate specific and have been selected during
CHOHO
CO2H
O
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
HOOH
HO
HOH2CO
OHC
CH2OPO32ndash
OHHO
CO2H
HO
OH
OH
TYR
PHE
CO2H
NH2
TRP
CH2OPO32ndash
HO
HOH2COH
MEP
C10 C20 and C40 terpenes
CO2HHO
SER
GLY CYS
CO2HOPO3
2ndash
CH2OH
OHHO2C
MVA
C15 and C30 terpenes
Polyketides
Krebscycle
(citric acid)
VAL ALA ILE LEU
CO2H
O
HO2C
CO2HO
CO2H
ASP
LYS
GLU
Glucose-6-phosphate
Fructose-6-phosphate
Erythrose-4-phosphate
3-PGAL
OP2O63ndash OP2O6
3ndash
IPP DMAPP
3-PGA PEP PA
O SCoAAcCoA
3x
Cytosol
Cytosol
Chloroplasts
Chloroplasts
Glycolysis
C2
C5
Shikimic acid Oxaloaceticacid
2-Oxoglutaricacid
MET
THR
PRO
ARGORN
Alkaloids Alkaloids Alkaloids Alkaloids
Aminoacids
Peptidesproteins
Phenolics
C1
C5N C4N
ldquoCH3rdquo
(Indole)C2NC6C3C6C2N C6C1
Pentosephosphatepathway
SCHEME 12 The building block chart involving glycolysis and the Krebs cycle
4 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
evolution to perform specific transformations making natural products with often and yet unknown functions
Secondary metabolites arise from specific biosynthetic pathways which use the previously defined building blocks The bunch of organic reactions involved in these biosynshytheses allows the construction of natural product frameshyworks which are finally diversified through ldquodecorationrdquo steps (Scheme 13) It is not the purpose of this introductive chapter to describe in detail all biosynthetic pathways and the reader can refer to excellent books and articles which have been published elsewhere [2 3]
The reactions involved in the construction of natural product skeletons will be described later for representative classes of compounds The identification of the building block footprint in the natural product skeleton will be emphasized as much as possible sometimes referring to biogenetic speculations [4] After the framework construction the decoration steps will involve as diverse reactions as aliphatic CH oxidations (eg involving a cytochrome P
450 oxygenase) occasionally triggering a
rearrangement heteroatom alkylations (eg methylation by S‐adenosylmethionine) or allylation (by DmAPP) esterifications heteroatom or C‐glycosylations (leading to heterosides) radical couplings (especially for phenols) alcohol oxidations or ketone reductions amineketone transaminations alkene dihydroxylations or epoxidations oxidative halogenations BaeyerndashVilliger oxidations and further oxygenation steps At the end of the biosynthesis such transformations may totally hide the primary building block origin of natural products
115 Secondary Metabolisms
1151 Polyketides Polyketides (or polyacetates) are issued from the oligomerization of C
2 acetate units performed
by polyketide synthases (PKS) and leading to (C2)
n linear
intermediates [5 6] If the (C2)
n intermediates arise from
successive Claisen reactions performed by ketosynthase
domains (KS in nonreducing PKS) a highly reactive poly‐ β‐ketoacyl intermediate H(CH
2CO)
nOH is formed
leading to phenolic and aromatic products through further intramolecular Claisen condensations Furthermore highly reducing PKSs are made of specialized enzymatic subunits working in line or iteratively to functionalize each C
2 linker
bond as CH(OH)CH2 (by ketoreductases (KR)) then as
HCCH (by dehydratases (DH)) and as CH2CH
2 (by enoyl
reductases (ER)) leading to a high degree of functionalization of the final product (Fig 12) By these iterative sequences highly reduced polyketides which can be either linear macrocyclized or polycyclized depending on the reactivity of the biosynthetic intermediates can be formed [7] With the same logic fatty acids are also biosynthesized by fatty acid synthases
moreover the PKS enzyme can be hybridized with nonshyribosomal peptide synthetase (NRPS) domains (see also ldquoNRPS metabolites and peptidesrdquo in the ldquoAlkaloidsrdquo secshytion) leading to the acylation of an amino acid by the (C
2)
n
acyl intermediate As previously the functionalization of the acyl chain depends on the PKS enzyme and the PKSNRPS products are also extremely diversified (eg hirsutellone B Fig 12) [8]
1152 Terpenes Terpenes are derived from the oligoshymerization of the C
5 isoprene units DmAPP and IPP Both
precursors are prompt to generate either an allylic cation (the diphosphate is a good leaving group) or a tertiary carbocation respectively which makes the IPP easy to react with DmAPP (Scheme 14) This reaction happens in the active site of a terpene synthase which activates the deparshyture of the diphosphate group from DmAPP thanks to Lewis acid activation (a metal like mg2+ mn2+ or Co2+ is present in the enzyme active site [9]) This leads to geranyl (C
10
monoterpene precursor) or farnesyl (C15
sesquiterpene precursor) diphosphate depending on the location of the enzyme (chloroplast for the mEP pathway or cytosol for the mVA pathway) Geranylgeranyl (C
20 diterpene) and
Constructionreactions
Decoration reaction(functionalization)Building
blocksNatural product
frameworksNatural products
HH
OH OAcH
HO O OH
OHO
OBz
P450
P450
P450
P450
P450 P450 Oxidativeauxiliaryenzymes
Taxadiene
OP2O63ndash
OP2O63ndash
3times
(a)
(b)
10-Deacetylbaccatin III
DMAPP
IPPand electrophilic
cyclizations
SCHEME 13 (a) From building blocks to natural products and (b) the example of 10‐deacetylbaccatin III
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 5
farnesylfarnesyl (C30
triterpene precursor) diphosphates can also be obtained by further additions of IPP leading to longer linear intermediates
The cyclization of linear precursors is achieved by speshycialized cyclases which generate a poorly functionalized natural product framework [10 11] Auxiliary enzymes such as oxygenases then increase the complexity and the diversity
of compounds by further functionalization (Scheme 13b) [12] A high degree of oxidation can be observed in comshypounds like thapsigargin paclitaxel or bilobalide (Fig 13) The biosynthesis of this last compound for example involves a high oxygenation pattern two Wagnerndashmeerwein rearrangements and several oxidative cleavages leading to the loss of five carbons The resulting natural products can
KS
S-ACP
O O
KR
S-ACP
OH O
S-MAT
O
YesNo
R
R
R
YesNoDH
S-ACP
OR
YesNoER
S-ACP
OR
YesNoTE
OH
OR
New cycle
New cycleor
release
New cycleor
release
New cycleor
release
Release
R in
crem
ente
d by
two
carb
ons
HO
O
S-ACP
O
Claisen condensation (ndashCO2)
reduction (NADPH)
dehydration (ndashH2O)
+
reduction (NADPH)
hydrolysis (H2O)
O O NH
OH
OH
H H
H H
Hirsutellone B(mixed PKSNRPS
product)
OO
O
OHOH
HO OH
Norsolorinic acid
O
O
O
O CHO
OH
HH
H
H
HH
OHHemibrevetoxin B
OOHO
OHO
HOHO OH
Erythronolide A
OH
OH
HO
Panaxytriol
From ahexanoylstartingblock
H
H
Fromtyrosine
H
1C lost fromdecarboxylation
FIGURE 12 Chemical logic of polyketide construction leading to variable functionalization of the elongated acyl chain and examples of resulting chemical diversity ACP acyl carrier protein DH dehydratase ER enoyl reductase KR ketoreductase KS ketosynthase mAT malonyl acyl transferase TE thioesterase
DMAPPOP2O6
3ndashOP2O6
3ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
Geranyl diphosphate (GPP)2 times IPP
H H
Geranylgeranyl diphosphate (GGPP)
Examplesverticillane
casbanetaxane
phorbol
Exampleslabdane
pimaranekauraneabietane
aphidicolanegibberellane
IPP
SCHEME 14 Early assembly of C5 units in terpene biosynthesis leading to diterpenes (C
20)
6 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
thus be extremely modified with structures whose biogenetic origin is far from being obvious at first sight and cannot be determined without further experiments such as isotopic labeling
1153 Flavonoids Resveratrols Gallic Acids and Further Polyphenolics We have previously discussed the polyketide origin of some phenolic compounds based on the (C
2)
n motif Other polyphenols like gallic acids are directly
derived by the aromatization of shikimic acid (C6C
1 building
block Scheme 12) [13] The C6C
3 building blocks are availshy
able from the conversion of phenylalanine and tyrosine into cinnamic and p‐coumaric acids respectively and then by further hydroxylation steps (Scheme 15) These can dimerize into lignans (eg podophyllotoxin) [14 15] through radical processes or converted to low molecular weight compounds like eugenol coumarins or vanillin [16] The coenzyme A thioesters of these C
6C
3 acids can be used as initiator units
by specialized ketosynthases for an elongation by two acetyl units leading to aromatic polyketides like styrylpyrones or diarylheptanoids (eg curcumin) [17] Important comshypounds from this metabolism are flavonoids (C
6C
3C
6) [18]
and stilbenoids (C6C
2C
6) (a decarboxylation occurs during
the aryl cyclization) [19] which are synthesized by chalcone synthase and stilbene synthase respectively Flavonoids (eg catechin) and stilbenes (eg resveratrol) are present in large amounts in fruits and vegetables and may exert their radical scavenging properties in vivo
1154 Alkaloids Alkaloids are nitrogen‐containing compounds The nitrogen(s) can be involved in an amine function conferring basicity to the natural product (like ldquoalkalirdquo) or in less or nonbasic functions such as an amide a nitrile an isonitrile or an ammonium salt (quaternary amines) For amines protonation often occurs at physiological pH and may condition their biological activity In many cases the nitrogen is biogenetically derived from an amino acid We will thus discuss alkaloids according to their amino acid origin
Alkaloids Derived from the Krebs Cycle (Lysine and Ornithine Derived) As shown previously (Scheme 12) the Krebs cycle is a crucial metabolic process which leads to α‐ketoacids (oxaloacetic and 2‐oxoglutaric acids) Their enzymatic transamination affords the two amino acidsmdashaspartic acid and glutamic acidmdashwhich are the direct biosynthetic precursors of amino acids lysine and ornithine respectively These in turn produce cadaverine a ldquoC
5Nrdquo unit
and putrescine a ldquoC4Nrdquo unit which are major components
for the biosynthesis of important alkaloids as will be discussed later (Scheme 16) Additionally ornithine is a precursor of arginine another important amino acid
ornithine‐derived alkaloids (incorporating the c4n
unit) Putrescine is derived from the decarboxylation of ornithine and is a precursor of linear polyamines like spermshyine After enzymatic methylation of one amine of putrescine
C10 C15
C15
C10
C10
C30
CO2H
Chrysanthemic acid
OH
Menthol
O
HO
H
HOGlc
MeO2CLoganin (iridoid)
H
H
H
HH
OO
OParthenolide
O
O
OHOHO
OAcHO
OnC3H7
O
O
O
nC7H15
Thapsigargin
O
O
OO
H
H
O
Cleavage
H
H
Artemisinin
C20 C40
O
O
OO
OO
OHOHH
Bilobalide(highly rearrangedand missing 5 C)
CleavageH
Highoxidativecleavages
HO OAcH
AcO O OH
OOOBz
O
Ph
BzNH
OHFrom PHE Paclitaxel
C25O
H
HO
OHCH
OH Ophiobolin A
H
HO
H
H
HHCholesterol
(missing 3 C WM)
H
H H H
H
H
Hopene
β-Carotene
C30 - 3
C15
C20 - 5WM
WM
FIGURE 13 Chemical diversity in the terpene series (Wm Wagnerndashmeerwein shifts lost carbons bold bonds are remnant of primary building blocks)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 7
in the presence of S‐adenosylmethionine transamination of the other affords γ‐(N‐methylamino)aldehyde [20] The resulting cyclic iminium is a key intermediate in the formation of many medicinally important alkaloids such as
the plant‐derived compounds cocaine atropine or the calysshytegines [21 22] Indeed this iminium is a mannich acceptor which can react with various nucleophiles the first of those being the carbanion of acetyl‐CoA Thus after a stepwise
CO2H
RCinnamic acid (R=H)p-Coumaric acid (R=OH)
RO
PHETYR
OH
[H] [O]
lignans
OH
O
O
O
O
OMeMeO
OMe
C mdash C andor C mdash Oradical couplings
After E Zisomerization
for example Podophyllotoxin
O ORO
Coumarins(eg scopoletin)
[O]
Prenylationcyclizationcleavage[O]
O OO
Furocoumarins(eg psoralen)
RO
nAcetyl-CoA
thencyclization
n = 2 styrylpyrones
O O
OMe
n = 3 avonoids(from chalcone synthase)
C6C3
H
H H
From AcCoA
O
n = 3 stilbenoids(from stilbene synthase)
HO
OH
OHFrom AcCoA From AcCoA(ndashCO2)
Resveratrol
HO
OH
OH
OH
OH
H
Catechin
MeO Yangonin
From AcCoA
SCHEME 15 The phenylpropanoid biosynthetic pathways
LYS
CO2
Cadaverine
O NH
H2O
NH
Piperideineiminium
Pelletierine
AcAcCoA
CO2
NH
O
N
O
Pseudopelletierine
NH
Ph
OH
Ph
O
Lobeline
NH
δ+
δndash
N
H
H
N
H
OH
Lupinine
N
H N
H
(+)-Sparteine
N
NH
O (ndash)-Cytisine
NH
CO2H
Pipecolic acid
N
OHHO
OH
HO
H
Castanos permine
ORN
CO2
NH2 NH2 NH2 NH2
NH2
NH2
NH2
PutrescineO
NHMe
N-Methylpyrrolinium
2 timesAcCoA
NMe
ARG
n = 1 spermidinen = 2 homospermidine
NMe
O
HygrineCO2
CO2
MeN
O
[O]
TropinoneCO2
HNHO
OHOH
OH
Calystegine B2
MeN
OAtropine
O
Ph
HO
NMe
NMe
NMe
O
Cuscohygrine
HN
HN
NH2
( )n
O
n = 2
N
HHO
Retronecine
HH
H
H
H
HO
H H
H
SCHEME 16 Lysine‐ and ornithine‐derived alkaloid biosynthetic pathways (mind the structural similarities)
8 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
elongation by two AcCoA units either decarboxylation can occur leading to the acetonylpyrrolidine hygrine or a secshyond mannich reaction by the intramolecular attack of the acetoacetate anion onto an oxidation‐derived pyrrolinium leading to the tropane skeleton (tropinone) The acetoacetyl‐CoA intermediate can also react intermolecularly with another pyrrolinium cation leading to cuscohygrine after decarboxylation Finally the pyrrolizidine alkaloids [23] are derived from homospermidine which when submitted to terminal oxidative deamination leads to the bicyclic skelshyeton of retronecine and further Senecio alkaloids We can mention herein that ornithine is a biosynthetic precursor of arginine bearing a guanidine function which is an intermediate toward the toxic compounds tetrodotoxin and saxitoxin (not shown)
lysine‐derived alkaloids (incorporating the c5n
unit) From lysine to piperidine alkaloids the biosynthetic steps parallel the one previously described from ornithine Indeed the oxidative deamination of cadaverine affords a δ‐amino aldehyde which cyclizes through imine formation into piperideine Protonation results in a mannich acceptor which is able to react with various nucleophiles such as β‐ketothioester anions The first product of these reactions is pelletierine which can further react through an intramolecular mannich
reaction leading to pseudopelletierine Quinolizidines [24] can also be formed first from the mannich reaction of the piperideine acceptor with the corresponding enamine nucleoshyphile and then after additional transformation steps leading for example to lupinine sparteine or cytisine
Indolizidine alkaloids [15] such as castanospermine and swainsonine are formed from pipecolic acid an amino acid derived from lysine which can be elongated by malonyl‐CoA followed by ring closure When protonated these alkaloids are oxonium mimics strongly inhibiting glycosidases
Tyrosine‐ and Phenylalanine‐Derived Alkaloids Tyrosine and phenylalanine amino acids are bearing the phenylshyethylamine moiety of many medicinally relevant alkaloids Further hydroxylations on the aromatic carbocycle or on the aliphatic part can be observed methylations can occur on phenolic oxygens and on the amine leading to catecholamines (adrenaline noradrenaline dopamine) Arylethylamines are also usual to react with endogenous aldehydes through PictetndashSpengler reactions [25] leading to important biosynthetic intermediates (Scheme 17) like
bull Reticuline from the reaction with 4‐hydroxyphenylacetshyaldehyde toward benzyltetrahydroisoquinoline alkaloids
NH2
Phenylethylamines
RONH TetrahydroisoquinolinesRO
RʹCHO
Rʹ
NH
(CH2)n
MeO
HO
Benzyltetrahydroisoquinoline(n = 1) for example reticuline
orPhenethyltetrahydroisoquinoline
(n = 2) eg autumnaline
IntermolecularCmdashO phenol
couplings
Curarealkaloids
IntramolecularC mdash C phenol
couplings
ONMe
HHO
H
HO
Morphine
NMe
MeO
HO
MeOOH
Isoboldine
O
O
OMe
NO2
CO2H
Aristolochic acid
OMe
O
NHAcMeO
MeOMeO
Colchicine
N
O
O
OMe
OMeBerberine
IntramolecularCmdash C coupling
and furtheroxidative events
Rʹ derivedfrom PHE
Pictet-Spenglerreaction
TYR
HO
HO CHO
HNHO
HO
OH
OMeO
OH
NMe
[O]
GalantamineNorbelladine
Rʹ derived fromsecologanin
NAc
HO
HO
O
CO2MeH
H
H
OHIpecosideaglycone
Ipecac alkaloids
1ClostCmdash C cleavage
and ring extension
H
ROH
1C lost
Cleavage
SCHEME 17 Tyrosine‐derived alkaloid biosynthetic pathways (double head arrows figure bond cleavages during biosynthetic processes)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 9
morphine berberine tubocurarine isoboldine or the highly modified aristolochic acid [26 27]
bull Automnaline from the reaction with 3‐(4‐hydroxyphenyl)propanal toward phenylethyltetrahydroisoquinoline alkaloids colchicine cephalotaxine or schelhammerishycine [28 29]
bull Ipecoside from the reaction of dopamine with secoloshyganin toward terpene tetrahydroisoquinoline alkaloids ipecoside or emetine
Lastly norbelladine (top of Scheme 17) is issued from the reductive amination of 34‐dihydroxybenzaldehyde (derived from phenylalanine) with tyramine (derived from tyrosine) and constitutes a biosynthetic node leading to Amaryllidaceae alkaloids such as galantamine crinine or lycorine depending on the topology of phenolic couplings In all these biosynthetic routes radical phenolic couplings are key reactions for CC and CO bond formations and rearrangements [30 31]
Tryptophan‐Derived Indole and Indole Monoterpene Alkaloids As for alkaloids derived from tyrosine and phenylalanine those derived from tryptophan are formed after decarboxylation of the amino acid (into tryptamine) and possible hydroxylation of the aromatic carbocycle (eg serotonin) and N‐methylation (eg psilocin) As previously tryptamine can also react through PictetndashSpengler reactions to form tetrahydro‐β‐carbolines which can be aromatized for example into harmine (Scheme 18) [16]
When the aldehyde partner of the PictetndashSpengler reacshytion with tryptamine is the terpene secologanin strictosidine is formed as an entry toward the vast monoterpene indole alkaloids [32 33] Hydrolysis of the glucosidic part releases the strictosidine aglycone bearing an aldehyde while iminshyium formation and further cyclization and reduction can lead to ajmalicine (from oxocyclization) or yohimbine (from carshybocyclization) These alkaloids are referred to as from the Corynanthe type with the monoterpene carbon skeleton unmodified Although it misses one carbon and has a very
RO
Carbonylcompounds
TRPNH
NH2
Indolethylamines(eg serotonine)
RONH
NH
Rʹ NH
N
MeOPictet-Spenglerreaction for example Harmineβ-Carbolines
NH
NHH
O
MeO2C
MeO2C
OH
H
H
Rʹ derived from secologanin
Strictosidine aglycone
NH
N
OH
H
H
Ajmalicine
N
N
O
O
H
H
H
Strychnine (C lost)Corynanthe type
Aspidosperma type Iboga type
N
N
OH
OMe
Quinine (C lost)
N
NH CO2Me
Catharanthine
NH
N
CO2MeH
OAc
OH
Vindoline
Complextransformations
NH
NMe
HN
MeN
H
H
Chimonanthine
Cyclizationdimerization
Prenylationcyclization
NH
NMeHO2C
H
Lysergic acid
H
H
Ergot alkaloidsPyrroloindoles Indole monoterpene
1C lost
1C lostFrom AcCoA
SCHEME 18 Tryptophan‐derived alkaloid biosynthetic pathways (gray parts monoterpenic units)
10 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
different structure strychnine is related to the Corynanthe alkaloids incorporating two carbons from acetyl‐CoA Highly modified monoterpene skeletons are derived from the Corynanthe core through CC bond breaking and reorshyganization leading to Iboga‐type (eg catharanthine) and Aspidosperma‐type (eg vindoline) alkaloids The antishycancer drug vinblastine is a heterodimer resulting from the nucleophilic attack of vindoline on a mannich acceptor resulting from catharanthine found in madagascar perishywinkle (Catharanthus roseus) The heteroaromatic comshypounds ellipticine camptothecin and quinine are also derived from a Corynanthe‐type precursor although in this case the biosynthetic relationship may not be obvious due to deep modifications of the skeleton
Finally two important classes of compounds have to be mentioned since they have inspired many synthetic chemists The pyrroloindole alkaloids result from the cyclization of tryptamine as found in physostigmine (formed by a cationic mechanism after methylation in position 3 of the indole not shown) or in chimonanthine (presumably formed by a radical coupling mechanism Scheme 18) The ergot alkaloids are derived from the 33‐dimethylallylation on position 4 of the indole in trypshytophan whose further cyclization and oxidation processes afford the natural products (eg lysergic acid Scheme 18 and ergotamine) which have had important medical applications [34]
NRPS Metabolites and Peptides NRPS enzymes assemble amino acids including nonproteinogenic ones into oligopeptides The enzymes contain several modules and especially an adenylation domain (A) which specifically selects and activates the amino acid to be transferred as a thioester on the nearby peptidyl carrier protein (PCP) [2] A condensation module (C) then catalyzes the formation of the peptide bonds between the newly introduced amino acyl‐PCP (bearing a free amine) and the elongated peptidyl‐PCP thioester At the end of the elongation a cyclization can occur into cyclopeptides but the peptide can also be
transferred to auxiliary enzymes like methyltransferases glycosyltransferases or oxidases (vancomycins are typical products of such functionalizations) [35 36] The formation of heterocycles is also frequently encountered in this metabolism as in penicillins that are derived from the tripeptide α‐aminoadipoyl‐cysteinyl‐valine or telomestatin (Fig 14) [2]
Other Alkaloid Origins There are many other nitrogen sources involved in alkaloid biosyntheses for example nicotinic acid (originated from aspartic acid and intershymediate in nicotine and anabasine biosyntheses) and anthranilic acid (originated from tryptophan and intermedishyate toward acridines or aurachins) The amination reaction (eg through transamination of carbonyl compounds) is also a way to introduce nitrogens in natural products for example from fatty acids steroids (toward Solanum alkaloids or cyclopamines) or other terpenoids (aconitine and atisine have diterpene skeletons while Daphniphyllum alkaloids are triterpene derivatives) Finally nucleic acids can also be precursors of alkaloids like the well‐known caffeine
12 FROM BIOSYNTHESIS TO TOTAL SYNTHESIS STRATEGIES TOWARD THE NATURAL PRODUCT CHEMICAL SPACE
121 The Chemical Space of Natural Products
Natural products occupy an important place in human com munities as demonstrated by their vast use from ancient times to nowadays like dyes fibers oils pershyfumes agrochemicals or drugs Broadly both primary and secondary metabolites could be classified as natural products while the latter as discussed previously are usushyally regarded as the ldquonatural productsrdquo owing to their comshyplexity and diversity arising from a variety of biosynthetic pathways The structural chemical diversity found among
N
S
CO2HO
HNPhH2C
O
Penicillin G
OH
HN
HN
O
O
ONH2
O
OHO
NHMe
OCl
O
NH
NH
NH
O
ClHO
O
HN
H
H
HOOH
OHHO2C
Vancomycin aglycone
ON
NO
O
NN
O
O
N
N O
Me
S
N N
OMeH
Telomestatin
L-AAA-L-CYS-D-VAL
Enzymes
FIGURE 14 Structural diversity of nonribosomal peptide compounds (AAA α‐aminoadipic acid)
FROm BIOSyNTHESIS TO TOTAL SyNTHESIS STRATEGIES TOWARD THE NATuRAL PRODuCT CHEmICAL SPACE 11
all living organisms defining the chemical space of natural products [37] is the consequence of their evolution occurshyring as an adaptation of organisms to their environment It is commonly believed that secondary metabolites are proshyduced as messengers by living organisms or as weapons against enemies and thus they should have certain biological activities in a medicinal point of view [38] Indeed natural products are regarded as one of the main sources of medicines (Fig 15) From the traditional medicinal extracts to every single bioactive molecule the methods of extraction purification identification and biological investigation of natural products have been well established Their complex structures and interesting properties have attracted synthetic chemists to accomplish their total syntheses and that of medicinally relevant anashylogs sometimes in the industrial context [39] Thus tarshygeting the chemical space of natural products has never been more relevant than today Although the discovery of natural products demands time and labor‐consuming manipulations it is worth to notice that the knowledge on this chemical space is still continually growing while biological advances allow for discovering and undershystanding potential targets However increasing the chemical space of human‐made compounds based on natural products should benefit from transdisciplinary colshylaborations such as the use of coupled biosynthetic and chemical synthetic methods to design original ldquounnatural natural productsrdquo [40]
122 The Biosynthetic Pathways as an Inspiration for Synthetic Challenges
1221 Precursor‐Directed Biosyntheses and Mutashysynthetic Strategies to Increase the Chemical Space of Natural Products As the genetics and biochemistry of natural product biosynthesis are better understood novel biosynthetic techniques have been developed to study and generate new diversity in natural product analogs (Scheme 19) Precursor‐directed biosynthesis (PDB) is considered as the earliest example of combining chemical and biological methods for the generation of complex natural product analogs [41 42] This approach compared with the biosynthetic pathway of wild‐type metabolites (Scheme 19a) involves the feeding of analogs of the natural biosynthetic building blocks to the living organisms (Scheme 19b) usually bacteria or fungi which incorporate the modified precursors into the biosynthesized compound mutasynthesis also termed as mutational biosynthesis (mBS) involves the inactivation of a key step of the bioshysynthesis in a mutant microorganism (Scheme 19c) which can then be fed by various modified or advanced building blocks (mutasynthons Scheme 19d) [43] These mutasshyynthons could not be incorporated by the wild type due to specificities of the enzymatic machinery Build up on PDB mBS eliminates the natural biosynthetic intermediate thus generating a less complex mixture of metabolites and making the purification or yield of target products better
O NH
OHO
O
OO
O
O
OH
HOO
O O
OH
HOO
NH ONH2
O
ONH
NH
O
OCl Cl
O
O
OH
HN
HN
HN
HN
OO
HO
HN
OH
OHOH
O
O
HO
HO
OHO
O
OHH2N
O
OH3C
H
O
H
CH3
CH3
H
O O
NO
H
O
HO
O
NO
O
NH
HN
OHO
HONH
ON
H
HO
O
H2N
O OH
ONH
OH
ON OHO
OH
HO OS OH
OOOH
OHO
O
ON
OO
O
HO
O
O OH
OO
Micafunginantifungal
Rapamycinimmunosuppressive
Galantamineanti-Alzheimer
Taxolanticancer
Artemisininantimalarial
Vancomycinantibiotic
FIGURE 15 Some famous natural products currently used as drugs
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product
viii CoNtENtS
22 Synthesis of Polyketides 44221 asymmetric alkylation Reactions 44222 applications of asymmetric alkylation Reactions in total Synthesis
of Polyketides and Macrolides 60References 8323 Synthesis of Polyketides‐Focus on Macrolides 87
231 Introduction 87232 Stereoselective Synthesis of 13‐Diols asymmetric aldol Reactions 88233 Stereoselective Synthesis of 13‐Diols asymmetric Reductions 106234 application of Stereoselective Synthesis of 13‐Diols in
the total Synthesis of Macrolides 117235 Conclusion 126
References 126
3 Fatty Acids and Their Derivatives 130Anders Vik and Trond Vidar Hansen
31 Introduction 13032 Biosynthesis 130
321 Fatty acids and Lipids 130322 Polyunsaturated Fatty acids 134323 Mediated oxidations of ω‐3 and ω‐6 Polyunsaturated
Fatty acids 13533 Synthesis of ω‐3 and ω‐6 all‐Z Polyunsaturated Fatty acids 140
331 Synthesis of Polyunsaturated Fatty acids by the Wittig Reaction or by the Polyyne Semihydrogenation 140
332 Synthesis of Polyunsaturated Fatty acids via Cross Coupling Reactions 143
34 applications in total Synthesis of Polyunsaturated Fatty acids 145341 Palladium‐Catalyzed Cross Coupling Reactions 145342 Biomimetic transformations of Polyunsaturated Fatty acids 149343 Landmark total Syntheses 153344 Synthesis of Leukotriene B
5 158
35 Conclusion 160acknowledgments 160References 160
4 Polyethers 162Youwei Xie and Paul E Floreancig
41 Introduction 16242 Biosynthesis 162
421 Ionophore antibiotics 162422 Marine Ladder toxins 165423 annonaceous acetogenins and terpene Polyethers 165
43 Epoxide Reactivity and Stereoselective Synthesis 166431 Regiocontrol in Epoxide‐opening Reactions 166432 Stereoselective Epoxide Synthesis 172
44 applications to total Synthesis 176441 acid‐Mediated transformations 176442 Cascades via Epoxonium Ion Formation 179443 Cyclizations under Basic Conditions 181444 Cyclization in Water 182
45 Conclusions 183References 184
CoNtENtS ix
SECTION II MEVALONATE BIOSYNTHETIC PATHWAY 187
5 From Acetate to Mevalonate and Deoxyxylulose Phosphate Biosynthetic Pathways An Introduction to Terpenoids 189Alexandros L Zografos and Elissavet E Anagnostaki
51 Introduction 18952 Mevalonic acid Pathway 19153 Mevalonate‐Independent Pathway 19254 Conclusion 194References 194
6 Monoterpenes and Iridoids 196Mario Waser and Uwe Rinner
61 Introduction 19662 Biosynthesis 196
621 acyclic Monoterpenes 197622 Cyclic Monoterpenes 197623 Iridoids 200624 Irregular Monoterpenes 202
63 asymmetric organocatalysis 203631 Introduction and Historical Background 204632 Enamine Iminium and Singly occupied Molecular
orbital activation 207633 Chiral (Broslashnsted) acids and H‐Bonding Donors 213634 Chiral BroslashnstedLewis Bases and Nucleophilic Catalysis 218635 asymmetric Phase‐transfer Catalysis 220
64 organocatalysis in the total Synthesis of Iridoids and Monoterpenoid Indole alkaloids 225641 (+)‐Geniposide and 7‐Deoxyloganin 226642 (ndash)‐Brasoside and (ndash)‐Littoralisone 227643 (+)‐Mitsugashiwalactone 229644 alstoscholarine 229645 (+)‐aspidospermidine and (+)‐Vincadifformine 230646 (+)‐Yohimbine 230
65 Conclusion 231References 231
7 Sesquiterpenes 236Alexandros L Zografos and Elissavet E Anagnostaki
71 Biosynthesis 23672 Cycloisomerization Reactions in organic Synthesis 244
721 Enyne Cycloisomerization 245722 Diene Cycloisomerization 257
73 application of Cycloisomerizations in the total Synthesis of Sesquiterpenoids 266731 Picrotoxane Sesquiterpenes 266732 aromadendrane Sesquiterpenes Epiglobulol 267733 CubebolndashCubebenes Sesquiterpenes 267734 Ventricos‐7(13)‐ene 270735 Englerins 271736 Echinopines 271737 Cyperolone 273
x CoNtENtS
738 Diverse Sesquiterpenoids 27674 Conclusion 276References 276
8 Diterpenes 279Louis Barriault
81 Introduction 27982 Biosynthesis of Diterpenes Based on Cationic Cyclizations
12‐Shifts and transannular Processes 27983 Pericyclic Reactions and their application in the Synthesis
of Selected Diterpenoids 284831 Dielsndashalder Reaction and Its application in the total
Synthesis of Diterpenes 284832 Cascade Pericyclic Reactions and their application in the total
Synthesis of Diterpenes 29184 Conclusion 293
References 294
9 Higher Terpenes and Steroids 296Kazuaki Ishihara
91 Introduction 29692 Biosynthesis 29693 Cascade Polyene Cyclizations 303
931 Diastereoselective Polyene Cyclizations 303932 ldquoChiral proton (H+)rdquo‐Induced Polyene Cyclizations 304933 ldquoChiral Metal Ionrdquo‐Induced Polyene Cyclizations 308934 ldquoChiral Halonium Ion (X+)rdquo‐Induced Polyene Cyclizations 313935 ldquoChiral Carbocationrdquo‐Induced Polyene Cyclizations 319936 Stereoselective Cyclizations of Homo(polyprenyl)arene
analogs 31994 Biomimetic total Synthesis of terpenes and Steroids through
Polyene Cyclization 31995 Conclusion 328
References 328
SECTION III SHIKIMIC ACID BIOSYNTHETIC PATHWAY 331
10 Lignans Lignins and Resveratrols 333Yu Peng
101 Biosynthesis 3331011 Primary Metabolism of Shikimic acid and aromatic
amino acids 3331012 Lignans and Lignin 335
102 auxiliary‐assisted C(sp3)ndashH arylation Reactions in organic Synthesis 336
103 FriedelndashCrafts Reactions in organic Synthesis 344104 total Synthesis of Lignans by C(sp3)H arylation Reactions 353105 total Synthesis of Lignans and Polymeric Resveratrol by
FriedelndashCrafts Reactions 357106 Conclusion 375References 375
CoNtENtS xi
SECTION IV MIXED BIOSYNTHETIC PATHWAYSndash THE STORY OF ALKALOIDS 381
11 Ornithine and Lysine Alkaloids 383Sebastian Brauch Wouter S Veldmate and Floris P J T Rutjes
111 Biosynthesis of l‐ornithine and l‐Lysine alkaloids 3831111 Biosynthetic Formation of alkaloids
Derived from l‐ornithine 3831112 Biosynthetic Formation of alkaloids
Derived from l‐Lysine 388112 the asymmetric Mannich Reaction in organic Synthesis 392
1121 Chiral amines as Catalysts in asymmetric Mannich Reactions 3941122 Chiral Broslashnsted Bases as Catalysts in asymmetric
Mannich Reactions 3981123 Chiral Broslashnsted acids as Catalysts in asymmetric
Mannich Reactions 4041124 organometallic Catalysts in asymmetric Mannich Reactions 4081125 Biocatalytic asymmetric Mannich Reactions 413
113 Mannich and Related Reactions in the total Synthesis of l‐Lysine‐ and l‐ornithine‐Derived alkaloids 414
114 Conclusion 426References 427
12 Tyrosine Alkaloids 431Uwe Rinner and Mario Waser
121 Introduction 431122 Biosynthesis of tyrosine‐Derived alkaloids 431
1221 Phenylethylamines 4311222 Simple tetrahydroisoquinoline alkaloids 4331223 Modified Benzyltetrahydroisoquinoline alkaloids 4331224 Phenethylisoquinoline alkaloids 4361225 amaryllidaceae alkaloids 4381226 Biosynthetic overview of tyrosine‐Derived alkaloids 442
123 arylndasharyl Coupling Reactions 4421231 Copper‐Mediated arylndasharyl Bond Forming Reactions 4431232 Nickel‐Mediated arylndasharyl Bond Forming Reactions 4461233 Palladium‐Mediated arylndasharyl Bond Forming Reactions 4471234 transition Metal‐Catalyzed Couplings of Nonactivated
aryl Compounds 450124 Synthesis of tyrosine‐Derived alkaloids 456
1241 Synthesis of Modified Benzyltetrahydroisoquinoline alkaloids 4561242 Synthesis of Phenethylisoquinoline alkaloids 4601243 Synthesis of amaryllidaceae alkaloids 462
125 Conclusion 468References 469
13 Histidine and Histidine‐Like Alkaloids 473Ian S Young
131 Introduction 473132 Biosynthesis 473133 atom Economy and Protecting‐Group‐Free Chemistry 480
xii CoNtENtS
134 Challenging the Boundaries of Synthesis PIas 488135 Conclusion 497References 499
14 Anthranilic AcidndashTryptophan Alkaloids 502Zhen‐Yu Tang
141 Biosynthesis 502142 Divergent SynthesisndashCollective total Synthesis 508143 Collective total Synthesis of tryptophan‐Derived alkaloids 510
1431 Monoterpene Indole alkaloids 5101432 Bisindole alkaloids 512
References 517
15 Future Directions of Modern Organic Synthesis 519Jakob Pletz and Rolf Breinbauer
151 Introduction 519152 Enzymes in organic Synthesis Merging total
Synthesis with Biosynthesis 520153 Engineered Biosynthesis 526154 Diversity‐oriented Synthesis Biology‐oriented Synthesis
and Diverted total Synthesis 5331541 Diversity‐oriented Synthesis 5351542 Biology‐oriented Synthesis 5361543 Diverted total Synthesis 539
155 Conclusion 541References 545
INDEX 548
Elissavet E Anagnostaki Department of Chemistry Laboratory of Organic Chemistry Aristotle University of Thessaloniki Thessaloniki Greece and Research and Development Department Pharmathen SA Thessaloniki Greece
Stellios Arseniyadis School of Biological and Chemical Sciences Queen Mary University of London London United Kingdom
Louis Barriault Department of Chemistry University of Ottawa Ottawa Ontario Canada
Erica Benedetti Laboratoire de Chimie et Biochimie et Pharmacologiques et Toxicologiques CNRS-Universiteacute Paris Descartes Faculteacute des Sciences Fondamentales et Biomeacutedicales Paris France
Sebastian Brauch Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
Rolf Breinbauer Institute of Organic Chemistry Technische Universitaumlt Graz Graz Austria
Cyril Bressy Aix Marseille Universiteacute Centrale Marseille CNRS Marseille France
Martin Cordes Institute for Organic Chemistry and Center of Biomolecular Drug Research (BMWZ) Leibniz Universitaumlt Hannover Hannover Germany and Helmholtz Center for Infection Research (HZI) Hannover Germany
Paul E Floreancig Department of Chemistry Chevron Science Center University of Pittsburgh Pittsburgh PA USA
Tobias A M Gulder Department of Chemistry and Center for Integrated Protein Science Munich (CIPSM) Biosystems Chemistry Technische Universitaumlt Muumlnchen Munich Germany
Trond Vidar Hansen School of Pharmacy University of Oslo Oslo Norway
Kazuaki Ishihara Department of Biotechnology Graduate School of Engineering Nagoya University Nagoya Japan
Markus Kalesse Institute for Organic Chemistry and Center of Biomolecular Drug Research (BMWZ) Leibniz Universitaumlt Hannover Hannover Germany and Helmholtz Center for Infection Research (HZI) Hannover Germany
Xu‐Wen Li Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
Bastien Nay Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France
Yu Peng State Key Laboratory of Applied Organic Chemistry Lanzhou University Lanzhou China
Jakob Pletz Institute of Organic Chemistry Technische Universitaumlt Graz Graz Austria
Uwe Rinner Institute of Organic Chemistry Johannes Kepler University Linz Linz Austria and Department of Chemistry College of Science Sultan Qaboos University Muscat Oman
Floris P J T Rutjes Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
LIST oF CoNTRIBUToRS
xiv LIST OF CONTRIBUTORS
Franccediloise Schaefers Department of Chemistry and Center for Integrated Protein Science Munich (CIPSM) Biosystems Chemistry Technische Universitaumlt Muumlnchen Munich Germany
Michael Smietana Institut des Biomoleacutecules Max Mousseron CNRS Universiteacute de Montpellier ENSCM France
Zhen‐Yu Tang Department of Pharmaceutical Engineering College of Chemistry and Chemical Engineering Central South University Changsha China
Wouter S Veldmate Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
Anders Vik School of Pharmacy University of Oslo Oslo Norway
Mario Waser Institute of Organic Chemistry Johannes Kepler University Linz Linz Austria
Youwei Xie Max‐Planck‐Institut fuumlr Kohlenforschung Muumllheim Germany
Ian S Young Bristol‐Myers Squibb Company Chemical Development New Brunswick NJ USA
Alexandros L Zografos Department of Chemistry Laboratory of Organic Chemistry Aristotle University of Thessaloniki Thessaloniki Greece
There is pleasure in the pathless woodsthere is rapture in the lonely shore
there is society where none intrudesby the deep sea and music in its roar
I love not Man the less but Nature moreLord Byron
Preface
The first time I came across with the idea of editing a book that merges selected chapters of biosynthesis and total synthesis was when I was teaching postgraduate courses of natural product synthesis at Aristotle University of Thessaloniki This period I realized that the best way to teach youngsters synthesis was to start from the very origin of inspiration nature and its tools biosynthesis
Over the last decades biosynthesis is filling our gaps of understanding the complex mechanisms of nature and pro-vides useful sources of inspiration not only in the way natural products can be synthesized but also by directing synthetic chemists in developing atom‐economical efficient synthetic methods Several are the examples that mimic biosynthetic guidelines from modern iterative alkylations and aldol reactions to CH oxidations that compile nowadays the modern toolbox of organic synthesis
The handed book is constructed in the logic of presenting the parallel development of biosynthesis and organic meth-odology and how these can be applied in efficient syntheses of natural products The book is divided into four sections each representing the four major biosynthetic pathways of natural products namely acetate mevalonate shikimate biosynthetic pathways and the mixed biosynthetic pathways
of alkaloids These sections are divided into chapters that represent selected classes of natural products for example lipids sesquiterpenoids lignans etc Each of these chapters is further divided into three distinct subchapters (a) biosyn-thesis (b) methodological section and (c) application of the described methodology in the total synthesis of the described family of natural products By this way the readers can be focused in the direct comparison between biosyn-theses and the developed methodologies to construct the crucial for each class of natural product carbon bonds Although the book as it develops is focused on presenting the power of biosynthesis and how this power can be applied in providing inspiration for the efficient synthesis of natural products it was not the authors will to present only biomi-metic total syntheses but rather to exploit the modern synthetic methodologies and recognize their disabilities for further improvement
Of course this book will not have been realized without the excellent work of renowned scientists worldwide working either in the field of biosynthesis or total synthesis who collected the existing knowledge on biosynthesis analyzed the existing modern methodologies and presented a bouquet of selected total syntheses Throughout our
xvi PrEfACE
endeavor to complete this book I learned many things from their expertise but I also realized that only with tight collab-orations you can build long‐lasting friendships I would like to thank them all once again for their trust and effort to complete this book We all hope that the current work will contribute to a better understanding of the current status of
organic chemistry and to the discovery of novel strategies and tactics for the synthesis of natural products
Alexandros L ZografosSeptember 2015
Thessaloniki Greece
From Biosynthesis to Total Synthesis Strategies and Tactics for Natural Products First Edition Edited by Alexandros L Zografoscopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
11 FROM PRIMARY TO SECONDARY METABOLISM THE KEY BUILDING BLOCKS
111 Definitions
The primary and secondary metabolisms are traditionally distinguished by their distribution and utility in the living organism network Primary metabolites include carbohyshydrates lipids nucleic acids and proteins (or their amino acid constituents) and are shared by all living organisms on Earth They are transformed by common pathways which are studied by biochemistry (Fig 11) Secondary metabo-lites are structurally diverse compounds usually produced by a limited number of organisms which synthesize them for a special purpose like defense or signaling through specific biosynthetic pathways They are studied by natural product chemistry This distinction is not always so obvious and some compounds can be studied in the context of both primary and secondary metabolisms This is especially true nowadays with the use of genetic and biomolecular tools which tend to make natural product sciences more and more integrative However an important point to remember is that the primary metabolism furnishes key building blocks to the secondary metabolism It would be difficult to describe in detail the full biosynthetic pathshyways in this section We tried to organize the discussion as a vade mecum synthetically gathering information from extremely useful sources which will be cited at the end of this chapter
112 Energy Supply and Carbon Storing at the Early Stage of Metabolisms
The sunlight is essential to life except in some part of the deep oceans It provides energy for plant photosynthesis that splits molecules of water into protons and electrons and releases O
2 (Scheme 11) A proton gradient inside the plant
chloroplasts then drags a transmembrane ATP synthase comshyplex that produces adenosine triphosphate (ATP) while elecshytrons released from water are transferred to the coenzyme reducer nicotinamide adenine dinucleotide phosphate hydride (NADPH) A major function of chloroplasts is to fix CO
2 as a combination to ribulose‐15‐bisphosphate (RuBP)
performed by RuBP carboxylase (rubisco) forming an instable ldquoC
6rdquo β‐ketoacid This is cleaved into two molecules
of 3‐phosphoglycerate (3‐PGA) which is then reduced into 3‐phosphoglyceraldehyde (3‐PGAL a ldquoC
3rdquo triose phosshy
phate) during the Calvin cycle This is one of the major metabolites in the biosynthesis of carbohydrates like glucose and a biochemical mean for storing and retaining carbon atoms in the living cells
113 Glucose as a Starting Material Toward Key Building Blocks of the Secondary Metabolism
Glucose‐6‐phosphate arises from the phosphorylation of glucose It is the starting material of glycolysis an important process of the primary metabolism which consists in eight enzymatic reactions leading to pyruvic acid (PA)
FROM BIOSYNTHESES TO TOTAL SYNTHESES AN INTRODUCTION
Bastien Nay1 and Xu‐Wen Li2
1
1 Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France2 Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
2 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
(Scheme 12) Important intermediates for the secondary metabolism are produced during glycolysis Glucose glucose‐6‐phosphate and fructose‐6‐phosphate can be converted to other hexoses and pentoses that can be oligoshymerized and enter in the composition of heterosides Additionally fructose‐6‐phosphate connects the pentose
phosphate pathway leading to erythrose‐4‐phosphate toward shikimic acid which is a key metabolite in the biosynthesis of aromatic amino acids (phenylalanine tyrosine or C
6C
3
units) and C6C
1 phenolic compounds The next important
intermediate in glycolysis is 3‐PGAL which can be redishyrected toward methylerythritol‐4‐phosphate (mEP) in the
Primary metabolism Secondary metabolism
The field ofbiochemistry
The field ofnatural product
chemistry
Essential toliving organisms
Essential to theproducer organisms
under particularconditions
Nucleic acids (DNARNA) carbohydrates
lipids amino acidspeptides and proteins
Alkaloids terpenespolyketides
polyphenols andtheir heterosidic form
Biological effects(defense signaling)
Biosyntheticpathways
Main compoundclasses
FIGURE 11 Primary versus secondary metabolisms
PS-I
PS-IIendash
hν H2O H+ and O2
ATP synthase
NADP reductase
H+ (gradient)
H+ADP + Pi ATP
NADPHH+NADP+
H+
Thylakoidcompartment
Chloroplaststroma
Thylakoidmembrane
(a) Light dependent process
(b) Light independent process
CO2
RuBP
Rubisco
Unstable β-ketoacid3-PGA 13-diPGA
ATP
ADP3-PGAL
NADPHH+
NADP+
(Calvin cycle)
trioses tetroses pentoses hexoses (eg glucose) heptoses
Chloroplaststroma
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
CH2OPO32ndash CH2OPO3
2ndash CH2OPO32ndash
CH2OPO32ndash
OOH
HO
HOO
HO
HO2CCO2H
HO
CO2PO32ndash
HO
CHOHO
endash
Cytosol
SCHEME 11 The photosynthetic machinery (PS‐I and PS‐II photosystems I and II)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 3
chloroplast mEP is a starting block in the biosynthesis of terpenes through C
5 isoprene units (isopentenyl diphosphate
(IPP) and dimethylallyl diphosphate (DmAPP)) especially those in C
10 C
20 and C
40 terpenes 3‐PGA is a precursor of
serine and other amino acids while phosphoenolpyruvate (PEP) the precursor of PA is also an intermediate toward the previously mentioned shikimic acid Lastly PA is not only a precursor of the fundamental ldquoC
2rdquo acetyl coenzyme A
(AcCoA) unit but also an intermediate toward aliphatic amino acids and mEP
AcCoA is the building block of fatty acids polyketides and mevalonic acid (mVA) a cytosolic precursor of the C
5 isoprene units for the biosynthesis of terpenes in the
C15
and C30
series (mind it is different from the mEP pathway in product and in cell location) Finally AcCoA enters the citric acid or Krebs cycle which leads to several precursors of amino acids These are oxaloacetic acid precursor of aspartic acid through transamination (thus toward lysine as a nitrogenated C
5N linear unit and methishy
onine as a methyl supplier) and 2‐oxoglutaric acid preshycursor of glutamic acid (and subsequent derivatives such as ornithine as a nitrogenated C
4N linear unit) All these
amino acids are key precursors in the biosynthesis of many alkaloids
114 Reactions Involved in the Construction of Secondary Metabolites
most reactions occurring in the living cells are performed by specialized enzymes which have been classified in an intershynational nomenclature defined by an enzyme commission (EC) number There are six classes of enzymes depending on the biochemical reaction they catalyze EC‐1 oxidoreducshytases (catalyzing oxidoreduction reactions) EC‐2 transfershyases (catalyzing the transfer of functional groups) EC‐3 hydrolases (catalyzing hydrolysis) EC‐4 lyases (breaking bonds through another process than hydrolysis or oxidation leading to a new double bond or a new cycle) EC‐5 isomershyases (catalyzing the isomerization of a molecule) and EC‐6 ligases (forming a covalent bond between two molecules) many subclasses of these enzymes have been described depending on the type of atoms and functional groups involved in the reaction and if any on the cofactor used in this reaction For example several cofactors can be used by dehydrogenases like NAD(P)NAD(P)H FADFADH
2 or
FmNFmNH2 For a description of this classification the
reader can refer to specialized Internet websites like ExplorEnz [1] What is important to realize is that most enzymes are substrate specific and have been selected during
CHOHO
CO2H
O
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
HOOH
HO
HOH2CO
OHC
CH2OPO32ndash
OHHO
CO2H
HO
OH
OH
TYR
PHE
CO2H
NH2
TRP
CH2OPO32ndash
HO
HOH2COH
MEP
C10 C20 and C40 terpenes
CO2HHO
SER
GLY CYS
CO2HOPO3
2ndash
CH2OH
OHHO2C
MVA
C15 and C30 terpenes
Polyketides
Krebscycle
(citric acid)
VAL ALA ILE LEU
CO2H
O
HO2C
CO2HO
CO2H
ASP
LYS
GLU
Glucose-6-phosphate
Fructose-6-phosphate
Erythrose-4-phosphate
3-PGAL
OP2O63ndash OP2O6
3ndash
IPP DMAPP
3-PGA PEP PA
O SCoAAcCoA
3x
Cytosol
Cytosol
Chloroplasts
Chloroplasts
Glycolysis
C2
C5
Shikimic acid Oxaloaceticacid
2-Oxoglutaricacid
MET
THR
PRO
ARGORN
Alkaloids Alkaloids Alkaloids Alkaloids
Aminoacids
Peptidesproteins
Phenolics
C1
C5N C4N
ldquoCH3rdquo
(Indole)C2NC6C3C6C2N C6C1
Pentosephosphatepathway
SCHEME 12 The building block chart involving glycolysis and the Krebs cycle
4 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
evolution to perform specific transformations making natural products with often and yet unknown functions
Secondary metabolites arise from specific biosynthetic pathways which use the previously defined building blocks The bunch of organic reactions involved in these biosynshytheses allows the construction of natural product frameshyworks which are finally diversified through ldquodecorationrdquo steps (Scheme 13) It is not the purpose of this introductive chapter to describe in detail all biosynthetic pathways and the reader can refer to excellent books and articles which have been published elsewhere [2 3]
The reactions involved in the construction of natural product skeletons will be described later for representative classes of compounds The identification of the building block footprint in the natural product skeleton will be emphasized as much as possible sometimes referring to biogenetic speculations [4] After the framework construction the decoration steps will involve as diverse reactions as aliphatic CH oxidations (eg involving a cytochrome P
450 oxygenase) occasionally triggering a
rearrangement heteroatom alkylations (eg methylation by S‐adenosylmethionine) or allylation (by DmAPP) esterifications heteroatom or C‐glycosylations (leading to heterosides) radical couplings (especially for phenols) alcohol oxidations or ketone reductions amineketone transaminations alkene dihydroxylations or epoxidations oxidative halogenations BaeyerndashVilliger oxidations and further oxygenation steps At the end of the biosynthesis such transformations may totally hide the primary building block origin of natural products
115 Secondary Metabolisms
1151 Polyketides Polyketides (or polyacetates) are issued from the oligomerization of C
2 acetate units performed
by polyketide synthases (PKS) and leading to (C2)
n linear
intermediates [5 6] If the (C2)
n intermediates arise from
successive Claisen reactions performed by ketosynthase
domains (KS in nonreducing PKS) a highly reactive poly‐ β‐ketoacyl intermediate H(CH
2CO)
nOH is formed
leading to phenolic and aromatic products through further intramolecular Claisen condensations Furthermore highly reducing PKSs are made of specialized enzymatic subunits working in line or iteratively to functionalize each C
2 linker
bond as CH(OH)CH2 (by ketoreductases (KR)) then as
HCCH (by dehydratases (DH)) and as CH2CH
2 (by enoyl
reductases (ER)) leading to a high degree of functionalization of the final product (Fig 12) By these iterative sequences highly reduced polyketides which can be either linear macrocyclized or polycyclized depending on the reactivity of the biosynthetic intermediates can be formed [7] With the same logic fatty acids are also biosynthesized by fatty acid synthases
moreover the PKS enzyme can be hybridized with nonshyribosomal peptide synthetase (NRPS) domains (see also ldquoNRPS metabolites and peptidesrdquo in the ldquoAlkaloidsrdquo secshytion) leading to the acylation of an amino acid by the (C
2)
n
acyl intermediate As previously the functionalization of the acyl chain depends on the PKS enzyme and the PKSNRPS products are also extremely diversified (eg hirsutellone B Fig 12) [8]
1152 Terpenes Terpenes are derived from the oligoshymerization of the C
5 isoprene units DmAPP and IPP Both
precursors are prompt to generate either an allylic cation (the diphosphate is a good leaving group) or a tertiary carbocation respectively which makes the IPP easy to react with DmAPP (Scheme 14) This reaction happens in the active site of a terpene synthase which activates the deparshyture of the diphosphate group from DmAPP thanks to Lewis acid activation (a metal like mg2+ mn2+ or Co2+ is present in the enzyme active site [9]) This leads to geranyl (C
10
monoterpene precursor) or farnesyl (C15
sesquiterpene precursor) diphosphate depending on the location of the enzyme (chloroplast for the mEP pathway or cytosol for the mVA pathway) Geranylgeranyl (C
20 diterpene) and
Constructionreactions
Decoration reaction(functionalization)Building
blocksNatural product
frameworksNatural products
HH
OH OAcH
HO O OH
OHO
OBz
P450
P450
P450
P450
P450 P450 Oxidativeauxiliaryenzymes
Taxadiene
OP2O63ndash
OP2O63ndash
3times
(a)
(b)
10-Deacetylbaccatin III
DMAPP
IPPand electrophilic
cyclizations
SCHEME 13 (a) From building blocks to natural products and (b) the example of 10‐deacetylbaccatin III
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 5
farnesylfarnesyl (C30
triterpene precursor) diphosphates can also be obtained by further additions of IPP leading to longer linear intermediates
The cyclization of linear precursors is achieved by speshycialized cyclases which generate a poorly functionalized natural product framework [10 11] Auxiliary enzymes such as oxygenases then increase the complexity and the diversity
of compounds by further functionalization (Scheme 13b) [12] A high degree of oxidation can be observed in comshypounds like thapsigargin paclitaxel or bilobalide (Fig 13) The biosynthesis of this last compound for example involves a high oxygenation pattern two Wagnerndashmeerwein rearrangements and several oxidative cleavages leading to the loss of five carbons The resulting natural products can
KS
S-ACP
O O
KR
S-ACP
OH O
S-MAT
O
YesNo
R
R
R
YesNoDH
S-ACP
OR
YesNoER
S-ACP
OR
YesNoTE
OH
OR
New cycle
New cycleor
release
New cycleor
release
New cycleor
release
Release
R in
crem
ente
d by
two
carb
ons
HO
O
S-ACP
O
Claisen condensation (ndashCO2)
reduction (NADPH)
dehydration (ndashH2O)
+
reduction (NADPH)
hydrolysis (H2O)
O O NH
OH
OH
H H
H H
Hirsutellone B(mixed PKSNRPS
product)
OO
O
OHOH
HO OH
Norsolorinic acid
O
O
O
O CHO
OH
HH
H
H
HH
OHHemibrevetoxin B
OOHO
OHO
HOHO OH
Erythronolide A
OH
OH
HO
Panaxytriol
From ahexanoylstartingblock
H
H
Fromtyrosine
H
1C lost fromdecarboxylation
FIGURE 12 Chemical logic of polyketide construction leading to variable functionalization of the elongated acyl chain and examples of resulting chemical diversity ACP acyl carrier protein DH dehydratase ER enoyl reductase KR ketoreductase KS ketosynthase mAT malonyl acyl transferase TE thioesterase
DMAPPOP2O6
3ndashOP2O6
3ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
Geranyl diphosphate (GPP)2 times IPP
H H
Geranylgeranyl diphosphate (GGPP)
Examplesverticillane
casbanetaxane
phorbol
Exampleslabdane
pimaranekauraneabietane
aphidicolanegibberellane
IPP
SCHEME 14 Early assembly of C5 units in terpene biosynthesis leading to diterpenes (C
20)
6 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
thus be extremely modified with structures whose biogenetic origin is far from being obvious at first sight and cannot be determined without further experiments such as isotopic labeling
1153 Flavonoids Resveratrols Gallic Acids and Further Polyphenolics We have previously discussed the polyketide origin of some phenolic compounds based on the (C
2)
n motif Other polyphenols like gallic acids are directly
derived by the aromatization of shikimic acid (C6C
1 building
block Scheme 12) [13] The C6C
3 building blocks are availshy
able from the conversion of phenylalanine and tyrosine into cinnamic and p‐coumaric acids respectively and then by further hydroxylation steps (Scheme 15) These can dimerize into lignans (eg podophyllotoxin) [14 15] through radical processes or converted to low molecular weight compounds like eugenol coumarins or vanillin [16] The coenzyme A thioesters of these C
6C
3 acids can be used as initiator units
by specialized ketosynthases for an elongation by two acetyl units leading to aromatic polyketides like styrylpyrones or diarylheptanoids (eg curcumin) [17] Important comshypounds from this metabolism are flavonoids (C
6C
3C
6) [18]
and stilbenoids (C6C
2C
6) (a decarboxylation occurs during
the aryl cyclization) [19] which are synthesized by chalcone synthase and stilbene synthase respectively Flavonoids (eg catechin) and stilbenes (eg resveratrol) are present in large amounts in fruits and vegetables and may exert their radical scavenging properties in vivo
1154 Alkaloids Alkaloids are nitrogen‐containing compounds The nitrogen(s) can be involved in an amine function conferring basicity to the natural product (like ldquoalkalirdquo) or in less or nonbasic functions such as an amide a nitrile an isonitrile or an ammonium salt (quaternary amines) For amines protonation often occurs at physiological pH and may condition their biological activity In many cases the nitrogen is biogenetically derived from an amino acid We will thus discuss alkaloids according to their amino acid origin
Alkaloids Derived from the Krebs Cycle (Lysine and Ornithine Derived) As shown previously (Scheme 12) the Krebs cycle is a crucial metabolic process which leads to α‐ketoacids (oxaloacetic and 2‐oxoglutaric acids) Their enzymatic transamination affords the two amino acidsmdashaspartic acid and glutamic acidmdashwhich are the direct biosynthetic precursors of amino acids lysine and ornithine respectively These in turn produce cadaverine a ldquoC
5Nrdquo unit
and putrescine a ldquoC4Nrdquo unit which are major components
for the biosynthesis of important alkaloids as will be discussed later (Scheme 16) Additionally ornithine is a precursor of arginine another important amino acid
ornithine‐derived alkaloids (incorporating the c4n
unit) Putrescine is derived from the decarboxylation of ornithine and is a precursor of linear polyamines like spermshyine After enzymatic methylation of one amine of putrescine
C10 C15
C15
C10
C10
C30
CO2H
Chrysanthemic acid
OH
Menthol
O
HO
H
HOGlc
MeO2CLoganin (iridoid)
H
H
H
HH
OO
OParthenolide
O
O
OHOHO
OAcHO
OnC3H7
O
O
O
nC7H15
Thapsigargin
O
O
OO
H
H
O
Cleavage
H
H
Artemisinin
C20 C40
O
O
OO
OO
OHOHH
Bilobalide(highly rearrangedand missing 5 C)
CleavageH
Highoxidativecleavages
HO OAcH
AcO O OH
OOOBz
O
Ph
BzNH
OHFrom PHE Paclitaxel
C25O
H
HO
OHCH
OH Ophiobolin A
H
HO
H
H
HHCholesterol
(missing 3 C WM)
H
H H H
H
H
Hopene
β-Carotene
C30 - 3
C15
C20 - 5WM
WM
FIGURE 13 Chemical diversity in the terpene series (Wm Wagnerndashmeerwein shifts lost carbons bold bonds are remnant of primary building blocks)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 7
in the presence of S‐adenosylmethionine transamination of the other affords γ‐(N‐methylamino)aldehyde [20] The resulting cyclic iminium is a key intermediate in the formation of many medicinally important alkaloids such as
the plant‐derived compounds cocaine atropine or the calysshytegines [21 22] Indeed this iminium is a mannich acceptor which can react with various nucleophiles the first of those being the carbanion of acetyl‐CoA Thus after a stepwise
CO2H
RCinnamic acid (R=H)p-Coumaric acid (R=OH)
RO
PHETYR
OH
[H] [O]
lignans
OH
O
O
O
O
OMeMeO
OMe
C mdash C andor C mdash Oradical couplings
After E Zisomerization
for example Podophyllotoxin
O ORO
Coumarins(eg scopoletin)
[O]
Prenylationcyclizationcleavage[O]
O OO
Furocoumarins(eg psoralen)
RO
nAcetyl-CoA
thencyclization
n = 2 styrylpyrones
O O
OMe
n = 3 avonoids(from chalcone synthase)
C6C3
H
H H
From AcCoA
O
n = 3 stilbenoids(from stilbene synthase)
HO
OH
OHFrom AcCoA From AcCoA(ndashCO2)
Resveratrol
HO
OH
OH
OH
OH
H
Catechin
MeO Yangonin
From AcCoA
SCHEME 15 The phenylpropanoid biosynthetic pathways
LYS
CO2
Cadaverine
O NH
H2O
NH
Piperideineiminium
Pelletierine
AcAcCoA
CO2
NH
O
N
O
Pseudopelletierine
NH
Ph
OH
Ph
O
Lobeline
NH
δ+
δndash
N
H
H
N
H
OH
Lupinine
N
H N
H
(+)-Sparteine
N
NH
O (ndash)-Cytisine
NH
CO2H
Pipecolic acid
N
OHHO
OH
HO
H
Castanos permine
ORN
CO2
NH2 NH2 NH2 NH2
NH2
NH2
NH2
PutrescineO
NHMe
N-Methylpyrrolinium
2 timesAcCoA
NMe
ARG
n = 1 spermidinen = 2 homospermidine
NMe
O
HygrineCO2
CO2
MeN
O
[O]
TropinoneCO2
HNHO
OHOH
OH
Calystegine B2
MeN
OAtropine
O
Ph
HO
NMe
NMe
NMe
O
Cuscohygrine
HN
HN
NH2
( )n
O
n = 2
N
HHO
Retronecine
HH
H
H
H
HO
H H
H
SCHEME 16 Lysine‐ and ornithine‐derived alkaloid biosynthetic pathways (mind the structural similarities)
8 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
elongation by two AcCoA units either decarboxylation can occur leading to the acetonylpyrrolidine hygrine or a secshyond mannich reaction by the intramolecular attack of the acetoacetate anion onto an oxidation‐derived pyrrolinium leading to the tropane skeleton (tropinone) The acetoacetyl‐CoA intermediate can also react intermolecularly with another pyrrolinium cation leading to cuscohygrine after decarboxylation Finally the pyrrolizidine alkaloids [23] are derived from homospermidine which when submitted to terminal oxidative deamination leads to the bicyclic skelshyeton of retronecine and further Senecio alkaloids We can mention herein that ornithine is a biosynthetic precursor of arginine bearing a guanidine function which is an intermediate toward the toxic compounds tetrodotoxin and saxitoxin (not shown)
lysine‐derived alkaloids (incorporating the c5n
unit) From lysine to piperidine alkaloids the biosynthetic steps parallel the one previously described from ornithine Indeed the oxidative deamination of cadaverine affords a δ‐amino aldehyde which cyclizes through imine formation into piperideine Protonation results in a mannich acceptor which is able to react with various nucleophiles such as β‐ketothioester anions The first product of these reactions is pelletierine which can further react through an intramolecular mannich
reaction leading to pseudopelletierine Quinolizidines [24] can also be formed first from the mannich reaction of the piperideine acceptor with the corresponding enamine nucleoshyphile and then after additional transformation steps leading for example to lupinine sparteine or cytisine
Indolizidine alkaloids [15] such as castanospermine and swainsonine are formed from pipecolic acid an amino acid derived from lysine which can be elongated by malonyl‐CoA followed by ring closure When protonated these alkaloids are oxonium mimics strongly inhibiting glycosidases
Tyrosine‐ and Phenylalanine‐Derived Alkaloids Tyrosine and phenylalanine amino acids are bearing the phenylshyethylamine moiety of many medicinally relevant alkaloids Further hydroxylations on the aromatic carbocycle or on the aliphatic part can be observed methylations can occur on phenolic oxygens and on the amine leading to catecholamines (adrenaline noradrenaline dopamine) Arylethylamines are also usual to react with endogenous aldehydes through PictetndashSpengler reactions [25] leading to important biosynthetic intermediates (Scheme 17) like
bull Reticuline from the reaction with 4‐hydroxyphenylacetshyaldehyde toward benzyltetrahydroisoquinoline alkaloids
NH2
Phenylethylamines
RONH TetrahydroisoquinolinesRO
RʹCHO
Rʹ
NH
(CH2)n
MeO
HO
Benzyltetrahydroisoquinoline(n = 1) for example reticuline
orPhenethyltetrahydroisoquinoline
(n = 2) eg autumnaline
IntermolecularCmdashO phenol
couplings
Curarealkaloids
IntramolecularC mdash C phenol
couplings
ONMe
HHO
H
HO
Morphine
NMe
MeO
HO
MeOOH
Isoboldine
O
O
OMe
NO2
CO2H
Aristolochic acid
OMe
O
NHAcMeO
MeOMeO
Colchicine
N
O
O
OMe
OMeBerberine
IntramolecularCmdash C coupling
and furtheroxidative events
Rʹ derivedfrom PHE
Pictet-Spenglerreaction
TYR
HO
HO CHO
HNHO
HO
OH
OMeO
OH
NMe
[O]
GalantamineNorbelladine
Rʹ derived fromsecologanin
NAc
HO
HO
O
CO2MeH
H
H
OHIpecosideaglycone
Ipecac alkaloids
1ClostCmdash C cleavage
and ring extension
H
ROH
1C lost
Cleavage
SCHEME 17 Tyrosine‐derived alkaloid biosynthetic pathways (double head arrows figure bond cleavages during biosynthetic processes)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 9
morphine berberine tubocurarine isoboldine or the highly modified aristolochic acid [26 27]
bull Automnaline from the reaction with 3‐(4‐hydroxyphenyl)propanal toward phenylethyltetrahydroisoquinoline alkaloids colchicine cephalotaxine or schelhammerishycine [28 29]
bull Ipecoside from the reaction of dopamine with secoloshyganin toward terpene tetrahydroisoquinoline alkaloids ipecoside or emetine
Lastly norbelladine (top of Scheme 17) is issued from the reductive amination of 34‐dihydroxybenzaldehyde (derived from phenylalanine) with tyramine (derived from tyrosine) and constitutes a biosynthetic node leading to Amaryllidaceae alkaloids such as galantamine crinine or lycorine depending on the topology of phenolic couplings In all these biosynthetic routes radical phenolic couplings are key reactions for CC and CO bond formations and rearrangements [30 31]
Tryptophan‐Derived Indole and Indole Monoterpene Alkaloids As for alkaloids derived from tyrosine and phenylalanine those derived from tryptophan are formed after decarboxylation of the amino acid (into tryptamine) and possible hydroxylation of the aromatic carbocycle (eg serotonin) and N‐methylation (eg psilocin) As previously tryptamine can also react through PictetndashSpengler reactions to form tetrahydro‐β‐carbolines which can be aromatized for example into harmine (Scheme 18) [16]
When the aldehyde partner of the PictetndashSpengler reacshytion with tryptamine is the terpene secologanin strictosidine is formed as an entry toward the vast monoterpene indole alkaloids [32 33] Hydrolysis of the glucosidic part releases the strictosidine aglycone bearing an aldehyde while iminshyium formation and further cyclization and reduction can lead to ajmalicine (from oxocyclization) or yohimbine (from carshybocyclization) These alkaloids are referred to as from the Corynanthe type with the monoterpene carbon skeleton unmodified Although it misses one carbon and has a very
RO
Carbonylcompounds
TRPNH
NH2
Indolethylamines(eg serotonine)
RONH
NH
Rʹ NH
N
MeOPictet-Spenglerreaction for example Harmineβ-Carbolines
NH
NHH
O
MeO2C
MeO2C
OH
H
H
Rʹ derived from secologanin
Strictosidine aglycone
NH
N
OH
H
H
Ajmalicine
N
N
O
O
H
H
H
Strychnine (C lost)Corynanthe type
Aspidosperma type Iboga type
N
N
OH
OMe
Quinine (C lost)
N
NH CO2Me
Catharanthine
NH
N
CO2MeH
OAc
OH
Vindoline
Complextransformations
NH
NMe
HN
MeN
H
H
Chimonanthine
Cyclizationdimerization
Prenylationcyclization
NH
NMeHO2C
H
Lysergic acid
H
H
Ergot alkaloidsPyrroloindoles Indole monoterpene
1C lost
1C lostFrom AcCoA
SCHEME 18 Tryptophan‐derived alkaloid biosynthetic pathways (gray parts monoterpenic units)
10 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
different structure strychnine is related to the Corynanthe alkaloids incorporating two carbons from acetyl‐CoA Highly modified monoterpene skeletons are derived from the Corynanthe core through CC bond breaking and reorshyganization leading to Iboga‐type (eg catharanthine) and Aspidosperma‐type (eg vindoline) alkaloids The antishycancer drug vinblastine is a heterodimer resulting from the nucleophilic attack of vindoline on a mannich acceptor resulting from catharanthine found in madagascar perishywinkle (Catharanthus roseus) The heteroaromatic comshypounds ellipticine camptothecin and quinine are also derived from a Corynanthe‐type precursor although in this case the biosynthetic relationship may not be obvious due to deep modifications of the skeleton
Finally two important classes of compounds have to be mentioned since they have inspired many synthetic chemists The pyrroloindole alkaloids result from the cyclization of tryptamine as found in physostigmine (formed by a cationic mechanism after methylation in position 3 of the indole not shown) or in chimonanthine (presumably formed by a radical coupling mechanism Scheme 18) The ergot alkaloids are derived from the 33‐dimethylallylation on position 4 of the indole in trypshytophan whose further cyclization and oxidation processes afford the natural products (eg lysergic acid Scheme 18 and ergotamine) which have had important medical applications [34]
NRPS Metabolites and Peptides NRPS enzymes assemble amino acids including nonproteinogenic ones into oligopeptides The enzymes contain several modules and especially an adenylation domain (A) which specifically selects and activates the amino acid to be transferred as a thioester on the nearby peptidyl carrier protein (PCP) [2] A condensation module (C) then catalyzes the formation of the peptide bonds between the newly introduced amino acyl‐PCP (bearing a free amine) and the elongated peptidyl‐PCP thioester At the end of the elongation a cyclization can occur into cyclopeptides but the peptide can also be
transferred to auxiliary enzymes like methyltransferases glycosyltransferases or oxidases (vancomycins are typical products of such functionalizations) [35 36] The formation of heterocycles is also frequently encountered in this metabolism as in penicillins that are derived from the tripeptide α‐aminoadipoyl‐cysteinyl‐valine or telomestatin (Fig 14) [2]
Other Alkaloid Origins There are many other nitrogen sources involved in alkaloid biosyntheses for example nicotinic acid (originated from aspartic acid and intershymediate in nicotine and anabasine biosyntheses) and anthranilic acid (originated from tryptophan and intermedishyate toward acridines or aurachins) The amination reaction (eg through transamination of carbonyl compounds) is also a way to introduce nitrogens in natural products for example from fatty acids steroids (toward Solanum alkaloids or cyclopamines) or other terpenoids (aconitine and atisine have diterpene skeletons while Daphniphyllum alkaloids are triterpene derivatives) Finally nucleic acids can also be precursors of alkaloids like the well‐known caffeine
12 FROM BIOSYNTHESIS TO TOTAL SYNTHESIS STRATEGIES TOWARD THE NATURAL PRODUCT CHEMICAL SPACE
121 The Chemical Space of Natural Products
Natural products occupy an important place in human com munities as demonstrated by their vast use from ancient times to nowadays like dyes fibers oils pershyfumes agrochemicals or drugs Broadly both primary and secondary metabolites could be classified as natural products while the latter as discussed previously are usushyally regarded as the ldquonatural productsrdquo owing to their comshyplexity and diversity arising from a variety of biosynthetic pathways The structural chemical diversity found among
N
S
CO2HO
HNPhH2C
O
Penicillin G
OH
HN
HN
O
O
ONH2
O
OHO
NHMe
OCl
O
NH
NH
NH
O
ClHO
O
HN
H
H
HOOH
OHHO2C
Vancomycin aglycone
ON
NO
O
NN
O
O
N
N O
Me
S
N N
OMeH
Telomestatin
L-AAA-L-CYS-D-VAL
Enzymes
FIGURE 14 Structural diversity of nonribosomal peptide compounds (AAA α‐aminoadipic acid)
FROm BIOSyNTHESIS TO TOTAL SyNTHESIS STRATEGIES TOWARD THE NATuRAL PRODuCT CHEmICAL SPACE 11
all living organisms defining the chemical space of natural products [37] is the consequence of their evolution occurshyring as an adaptation of organisms to their environment It is commonly believed that secondary metabolites are proshyduced as messengers by living organisms or as weapons against enemies and thus they should have certain biological activities in a medicinal point of view [38] Indeed natural products are regarded as one of the main sources of medicines (Fig 15) From the traditional medicinal extracts to every single bioactive molecule the methods of extraction purification identification and biological investigation of natural products have been well established Their complex structures and interesting properties have attracted synthetic chemists to accomplish their total syntheses and that of medicinally relevant anashylogs sometimes in the industrial context [39] Thus tarshygeting the chemical space of natural products has never been more relevant than today Although the discovery of natural products demands time and labor‐consuming manipulations it is worth to notice that the knowledge on this chemical space is still continually growing while biological advances allow for discovering and undershystanding potential targets However increasing the chemical space of human‐made compounds based on natural products should benefit from transdisciplinary colshylaborations such as the use of coupled biosynthetic and chemical synthetic methods to design original ldquounnatural natural productsrdquo [40]
122 The Biosynthetic Pathways as an Inspiration for Synthetic Challenges
1221 Precursor‐Directed Biosyntheses and Mutashysynthetic Strategies to Increase the Chemical Space of Natural Products As the genetics and biochemistry of natural product biosynthesis are better understood novel biosynthetic techniques have been developed to study and generate new diversity in natural product analogs (Scheme 19) Precursor‐directed biosynthesis (PDB) is considered as the earliest example of combining chemical and biological methods for the generation of complex natural product analogs [41 42] This approach compared with the biosynthetic pathway of wild‐type metabolites (Scheme 19a) involves the feeding of analogs of the natural biosynthetic building blocks to the living organisms (Scheme 19b) usually bacteria or fungi which incorporate the modified precursors into the biosynthesized compound mutasynthesis also termed as mutational biosynthesis (mBS) involves the inactivation of a key step of the bioshysynthesis in a mutant microorganism (Scheme 19c) which can then be fed by various modified or advanced building blocks (mutasynthons Scheme 19d) [43] These mutasshyynthons could not be incorporated by the wild type due to specificities of the enzymatic machinery Build up on PDB mBS eliminates the natural biosynthetic intermediate thus generating a less complex mixture of metabolites and making the purification or yield of target products better
O NH
OHO
O
OO
O
O
OH
HOO
O O
OH
HOO
NH ONH2
O
ONH
NH
O
OCl Cl
O
O
OH
HN
HN
HN
HN
OO
HO
HN
OH
OHOH
O
O
HO
HO
OHO
O
OHH2N
O
OH3C
H
O
H
CH3
CH3
H
O O
NO
H
O
HO
O
NO
O
NH
HN
OHO
HONH
ON
H
HO
O
H2N
O OH
ONH
OH
ON OHO
OH
HO OS OH
OOOH
OHO
O
ON
OO
O
HO
O
O OH
OO
Micafunginantifungal
Rapamycinimmunosuppressive
Galantamineanti-Alzheimer
Taxolanticancer
Artemisininantimalarial
Vancomycinantibiotic
FIGURE 15 Some famous natural products currently used as drugs
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product
CoNtENtS ix
SECTION II MEVALONATE BIOSYNTHETIC PATHWAY 187
5 From Acetate to Mevalonate and Deoxyxylulose Phosphate Biosynthetic Pathways An Introduction to Terpenoids 189Alexandros L Zografos and Elissavet E Anagnostaki
51 Introduction 18952 Mevalonic acid Pathway 19153 Mevalonate‐Independent Pathway 19254 Conclusion 194References 194
6 Monoterpenes and Iridoids 196Mario Waser and Uwe Rinner
61 Introduction 19662 Biosynthesis 196
621 acyclic Monoterpenes 197622 Cyclic Monoterpenes 197623 Iridoids 200624 Irregular Monoterpenes 202
63 asymmetric organocatalysis 203631 Introduction and Historical Background 204632 Enamine Iminium and Singly occupied Molecular
orbital activation 207633 Chiral (Broslashnsted) acids and H‐Bonding Donors 213634 Chiral BroslashnstedLewis Bases and Nucleophilic Catalysis 218635 asymmetric Phase‐transfer Catalysis 220
64 organocatalysis in the total Synthesis of Iridoids and Monoterpenoid Indole alkaloids 225641 (+)‐Geniposide and 7‐Deoxyloganin 226642 (ndash)‐Brasoside and (ndash)‐Littoralisone 227643 (+)‐Mitsugashiwalactone 229644 alstoscholarine 229645 (+)‐aspidospermidine and (+)‐Vincadifformine 230646 (+)‐Yohimbine 230
65 Conclusion 231References 231
7 Sesquiterpenes 236Alexandros L Zografos and Elissavet E Anagnostaki
71 Biosynthesis 23672 Cycloisomerization Reactions in organic Synthesis 244
721 Enyne Cycloisomerization 245722 Diene Cycloisomerization 257
73 application of Cycloisomerizations in the total Synthesis of Sesquiterpenoids 266731 Picrotoxane Sesquiterpenes 266732 aromadendrane Sesquiterpenes Epiglobulol 267733 CubebolndashCubebenes Sesquiterpenes 267734 Ventricos‐7(13)‐ene 270735 Englerins 271736 Echinopines 271737 Cyperolone 273
x CoNtENtS
738 Diverse Sesquiterpenoids 27674 Conclusion 276References 276
8 Diterpenes 279Louis Barriault
81 Introduction 27982 Biosynthesis of Diterpenes Based on Cationic Cyclizations
12‐Shifts and transannular Processes 27983 Pericyclic Reactions and their application in the Synthesis
of Selected Diterpenoids 284831 Dielsndashalder Reaction and Its application in the total
Synthesis of Diterpenes 284832 Cascade Pericyclic Reactions and their application in the total
Synthesis of Diterpenes 29184 Conclusion 293
References 294
9 Higher Terpenes and Steroids 296Kazuaki Ishihara
91 Introduction 29692 Biosynthesis 29693 Cascade Polyene Cyclizations 303
931 Diastereoselective Polyene Cyclizations 303932 ldquoChiral proton (H+)rdquo‐Induced Polyene Cyclizations 304933 ldquoChiral Metal Ionrdquo‐Induced Polyene Cyclizations 308934 ldquoChiral Halonium Ion (X+)rdquo‐Induced Polyene Cyclizations 313935 ldquoChiral Carbocationrdquo‐Induced Polyene Cyclizations 319936 Stereoselective Cyclizations of Homo(polyprenyl)arene
analogs 31994 Biomimetic total Synthesis of terpenes and Steroids through
Polyene Cyclization 31995 Conclusion 328
References 328
SECTION III SHIKIMIC ACID BIOSYNTHETIC PATHWAY 331
10 Lignans Lignins and Resveratrols 333Yu Peng
101 Biosynthesis 3331011 Primary Metabolism of Shikimic acid and aromatic
amino acids 3331012 Lignans and Lignin 335
102 auxiliary‐assisted C(sp3)ndashH arylation Reactions in organic Synthesis 336
103 FriedelndashCrafts Reactions in organic Synthesis 344104 total Synthesis of Lignans by C(sp3)H arylation Reactions 353105 total Synthesis of Lignans and Polymeric Resveratrol by
FriedelndashCrafts Reactions 357106 Conclusion 375References 375
CoNtENtS xi
SECTION IV MIXED BIOSYNTHETIC PATHWAYSndash THE STORY OF ALKALOIDS 381
11 Ornithine and Lysine Alkaloids 383Sebastian Brauch Wouter S Veldmate and Floris P J T Rutjes
111 Biosynthesis of l‐ornithine and l‐Lysine alkaloids 3831111 Biosynthetic Formation of alkaloids
Derived from l‐ornithine 3831112 Biosynthetic Formation of alkaloids
Derived from l‐Lysine 388112 the asymmetric Mannich Reaction in organic Synthesis 392
1121 Chiral amines as Catalysts in asymmetric Mannich Reactions 3941122 Chiral Broslashnsted Bases as Catalysts in asymmetric
Mannich Reactions 3981123 Chiral Broslashnsted acids as Catalysts in asymmetric
Mannich Reactions 4041124 organometallic Catalysts in asymmetric Mannich Reactions 4081125 Biocatalytic asymmetric Mannich Reactions 413
113 Mannich and Related Reactions in the total Synthesis of l‐Lysine‐ and l‐ornithine‐Derived alkaloids 414
114 Conclusion 426References 427
12 Tyrosine Alkaloids 431Uwe Rinner and Mario Waser
121 Introduction 431122 Biosynthesis of tyrosine‐Derived alkaloids 431
1221 Phenylethylamines 4311222 Simple tetrahydroisoquinoline alkaloids 4331223 Modified Benzyltetrahydroisoquinoline alkaloids 4331224 Phenethylisoquinoline alkaloids 4361225 amaryllidaceae alkaloids 4381226 Biosynthetic overview of tyrosine‐Derived alkaloids 442
123 arylndasharyl Coupling Reactions 4421231 Copper‐Mediated arylndasharyl Bond Forming Reactions 4431232 Nickel‐Mediated arylndasharyl Bond Forming Reactions 4461233 Palladium‐Mediated arylndasharyl Bond Forming Reactions 4471234 transition Metal‐Catalyzed Couplings of Nonactivated
aryl Compounds 450124 Synthesis of tyrosine‐Derived alkaloids 456
1241 Synthesis of Modified Benzyltetrahydroisoquinoline alkaloids 4561242 Synthesis of Phenethylisoquinoline alkaloids 4601243 Synthesis of amaryllidaceae alkaloids 462
125 Conclusion 468References 469
13 Histidine and Histidine‐Like Alkaloids 473Ian S Young
131 Introduction 473132 Biosynthesis 473133 atom Economy and Protecting‐Group‐Free Chemistry 480
xii CoNtENtS
134 Challenging the Boundaries of Synthesis PIas 488135 Conclusion 497References 499
14 Anthranilic AcidndashTryptophan Alkaloids 502Zhen‐Yu Tang
141 Biosynthesis 502142 Divergent SynthesisndashCollective total Synthesis 508143 Collective total Synthesis of tryptophan‐Derived alkaloids 510
1431 Monoterpene Indole alkaloids 5101432 Bisindole alkaloids 512
References 517
15 Future Directions of Modern Organic Synthesis 519Jakob Pletz and Rolf Breinbauer
151 Introduction 519152 Enzymes in organic Synthesis Merging total
Synthesis with Biosynthesis 520153 Engineered Biosynthesis 526154 Diversity‐oriented Synthesis Biology‐oriented Synthesis
and Diverted total Synthesis 5331541 Diversity‐oriented Synthesis 5351542 Biology‐oriented Synthesis 5361543 Diverted total Synthesis 539
155 Conclusion 541References 545
INDEX 548
Elissavet E Anagnostaki Department of Chemistry Laboratory of Organic Chemistry Aristotle University of Thessaloniki Thessaloniki Greece and Research and Development Department Pharmathen SA Thessaloniki Greece
Stellios Arseniyadis School of Biological and Chemical Sciences Queen Mary University of London London United Kingdom
Louis Barriault Department of Chemistry University of Ottawa Ottawa Ontario Canada
Erica Benedetti Laboratoire de Chimie et Biochimie et Pharmacologiques et Toxicologiques CNRS-Universiteacute Paris Descartes Faculteacute des Sciences Fondamentales et Biomeacutedicales Paris France
Sebastian Brauch Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
Rolf Breinbauer Institute of Organic Chemistry Technische Universitaumlt Graz Graz Austria
Cyril Bressy Aix Marseille Universiteacute Centrale Marseille CNRS Marseille France
Martin Cordes Institute for Organic Chemistry and Center of Biomolecular Drug Research (BMWZ) Leibniz Universitaumlt Hannover Hannover Germany and Helmholtz Center for Infection Research (HZI) Hannover Germany
Paul E Floreancig Department of Chemistry Chevron Science Center University of Pittsburgh Pittsburgh PA USA
Tobias A M Gulder Department of Chemistry and Center for Integrated Protein Science Munich (CIPSM) Biosystems Chemistry Technische Universitaumlt Muumlnchen Munich Germany
Trond Vidar Hansen School of Pharmacy University of Oslo Oslo Norway
Kazuaki Ishihara Department of Biotechnology Graduate School of Engineering Nagoya University Nagoya Japan
Markus Kalesse Institute for Organic Chemistry and Center of Biomolecular Drug Research (BMWZ) Leibniz Universitaumlt Hannover Hannover Germany and Helmholtz Center for Infection Research (HZI) Hannover Germany
Xu‐Wen Li Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
Bastien Nay Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France
Yu Peng State Key Laboratory of Applied Organic Chemistry Lanzhou University Lanzhou China
Jakob Pletz Institute of Organic Chemistry Technische Universitaumlt Graz Graz Austria
Uwe Rinner Institute of Organic Chemistry Johannes Kepler University Linz Linz Austria and Department of Chemistry College of Science Sultan Qaboos University Muscat Oman
Floris P J T Rutjes Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
LIST oF CoNTRIBUToRS
xiv LIST OF CONTRIBUTORS
Franccediloise Schaefers Department of Chemistry and Center for Integrated Protein Science Munich (CIPSM) Biosystems Chemistry Technische Universitaumlt Muumlnchen Munich Germany
Michael Smietana Institut des Biomoleacutecules Max Mousseron CNRS Universiteacute de Montpellier ENSCM France
Zhen‐Yu Tang Department of Pharmaceutical Engineering College of Chemistry and Chemical Engineering Central South University Changsha China
Wouter S Veldmate Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
Anders Vik School of Pharmacy University of Oslo Oslo Norway
Mario Waser Institute of Organic Chemistry Johannes Kepler University Linz Linz Austria
Youwei Xie Max‐Planck‐Institut fuumlr Kohlenforschung Muumllheim Germany
Ian S Young Bristol‐Myers Squibb Company Chemical Development New Brunswick NJ USA
Alexandros L Zografos Department of Chemistry Laboratory of Organic Chemistry Aristotle University of Thessaloniki Thessaloniki Greece
There is pleasure in the pathless woodsthere is rapture in the lonely shore
there is society where none intrudesby the deep sea and music in its roar
I love not Man the less but Nature moreLord Byron
Preface
The first time I came across with the idea of editing a book that merges selected chapters of biosynthesis and total synthesis was when I was teaching postgraduate courses of natural product synthesis at Aristotle University of Thessaloniki This period I realized that the best way to teach youngsters synthesis was to start from the very origin of inspiration nature and its tools biosynthesis
Over the last decades biosynthesis is filling our gaps of understanding the complex mechanisms of nature and pro-vides useful sources of inspiration not only in the way natural products can be synthesized but also by directing synthetic chemists in developing atom‐economical efficient synthetic methods Several are the examples that mimic biosynthetic guidelines from modern iterative alkylations and aldol reactions to CH oxidations that compile nowadays the modern toolbox of organic synthesis
The handed book is constructed in the logic of presenting the parallel development of biosynthesis and organic meth-odology and how these can be applied in efficient syntheses of natural products The book is divided into four sections each representing the four major biosynthetic pathways of natural products namely acetate mevalonate shikimate biosynthetic pathways and the mixed biosynthetic pathways
of alkaloids These sections are divided into chapters that represent selected classes of natural products for example lipids sesquiterpenoids lignans etc Each of these chapters is further divided into three distinct subchapters (a) biosyn-thesis (b) methodological section and (c) application of the described methodology in the total synthesis of the described family of natural products By this way the readers can be focused in the direct comparison between biosyn-theses and the developed methodologies to construct the crucial for each class of natural product carbon bonds Although the book as it develops is focused on presenting the power of biosynthesis and how this power can be applied in providing inspiration for the efficient synthesis of natural products it was not the authors will to present only biomi-metic total syntheses but rather to exploit the modern synthetic methodologies and recognize their disabilities for further improvement
Of course this book will not have been realized without the excellent work of renowned scientists worldwide working either in the field of biosynthesis or total synthesis who collected the existing knowledge on biosynthesis analyzed the existing modern methodologies and presented a bouquet of selected total syntheses Throughout our
xvi PrEfACE
endeavor to complete this book I learned many things from their expertise but I also realized that only with tight collab-orations you can build long‐lasting friendships I would like to thank them all once again for their trust and effort to complete this book We all hope that the current work will contribute to a better understanding of the current status of
organic chemistry and to the discovery of novel strategies and tactics for the synthesis of natural products
Alexandros L ZografosSeptember 2015
Thessaloniki Greece
From Biosynthesis to Total Synthesis Strategies and Tactics for Natural Products First Edition Edited by Alexandros L Zografoscopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
11 FROM PRIMARY TO SECONDARY METABOLISM THE KEY BUILDING BLOCKS
111 Definitions
The primary and secondary metabolisms are traditionally distinguished by their distribution and utility in the living organism network Primary metabolites include carbohyshydrates lipids nucleic acids and proteins (or their amino acid constituents) and are shared by all living organisms on Earth They are transformed by common pathways which are studied by biochemistry (Fig 11) Secondary metabo-lites are structurally diverse compounds usually produced by a limited number of organisms which synthesize them for a special purpose like defense or signaling through specific biosynthetic pathways They are studied by natural product chemistry This distinction is not always so obvious and some compounds can be studied in the context of both primary and secondary metabolisms This is especially true nowadays with the use of genetic and biomolecular tools which tend to make natural product sciences more and more integrative However an important point to remember is that the primary metabolism furnishes key building blocks to the secondary metabolism It would be difficult to describe in detail the full biosynthetic pathshyways in this section We tried to organize the discussion as a vade mecum synthetically gathering information from extremely useful sources which will be cited at the end of this chapter
112 Energy Supply and Carbon Storing at the Early Stage of Metabolisms
The sunlight is essential to life except in some part of the deep oceans It provides energy for plant photosynthesis that splits molecules of water into protons and electrons and releases O
2 (Scheme 11) A proton gradient inside the plant
chloroplasts then drags a transmembrane ATP synthase comshyplex that produces adenosine triphosphate (ATP) while elecshytrons released from water are transferred to the coenzyme reducer nicotinamide adenine dinucleotide phosphate hydride (NADPH) A major function of chloroplasts is to fix CO
2 as a combination to ribulose‐15‐bisphosphate (RuBP)
performed by RuBP carboxylase (rubisco) forming an instable ldquoC
6rdquo β‐ketoacid This is cleaved into two molecules
of 3‐phosphoglycerate (3‐PGA) which is then reduced into 3‐phosphoglyceraldehyde (3‐PGAL a ldquoC
3rdquo triose phosshy
phate) during the Calvin cycle This is one of the major metabolites in the biosynthesis of carbohydrates like glucose and a biochemical mean for storing and retaining carbon atoms in the living cells
113 Glucose as a Starting Material Toward Key Building Blocks of the Secondary Metabolism
Glucose‐6‐phosphate arises from the phosphorylation of glucose It is the starting material of glycolysis an important process of the primary metabolism which consists in eight enzymatic reactions leading to pyruvic acid (PA)
FROM BIOSYNTHESES TO TOTAL SYNTHESES AN INTRODUCTION
Bastien Nay1 and Xu‐Wen Li2
1
1 Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France2 Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
2 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
(Scheme 12) Important intermediates for the secondary metabolism are produced during glycolysis Glucose glucose‐6‐phosphate and fructose‐6‐phosphate can be converted to other hexoses and pentoses that can be oligoshymerized and enter in the composition of heterosides Additionally fructose‐6‐phosphate connects the pentose
phosphate pathway leading to erythrose‐4‐phosphate toward shikimic acid which is a key metabolite in the biosynthesis of aromatic amino acids (phenylalanine tyrosine or C
6C
3
units) and C6C
1 phenolic compounds The next important
intermediate in glycolysis is 3‐PGAL which can be redishyrected toward methylerythritol‐4‐phosphate (mEP) in the
Primary metabolism Secondary metabolism
The field ofbiochemistry
The field ofnatural product
chemistry
Essential toliving organisms
Essential to theproducer organisms
under particularconditions
Nucleic acids (DNARNA) carbohydrates
lipids amino acidspeptides and proteins
Alkaloids terpenespolyketides
polyphenols andtheir heterosidic form
Biological effects(defense signaling)
Biosyntheticpathways
Main compoundclasses
FIGURE 11 Primary versus secondary metabolisms
PS-I
PS-IIendash
hν H2O H+ and O2
ATP synthase
NADP reductase
H+ (gradient)
H+ADP + Pi ATP
NADPHH+NADP+
H+
Thylakoidcompartment
Chloroplaststroma
Thylakoidmembrane
(a) Light dependent process
(b) Light independent process
CO2
RuBP
Rubisco
Unstable β-ketoacid3-PGA 13-diPGA
ATP
ADP3-PGAL
NADPHH+
NADP+
(Calvin cycle)
trioses tetroses pentoses hexoses (eg glucose) heptoses
Chloroplaststroma
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
CH2OPO32ndash CH2OPO3
2ndash CH2OPO32ndash
CH2OPO32ndash
OOH
HO
HOO
HO
HO2CCO2H
HO
CO2PO32ndash
HO
CHOHO
endash
Cytosol
SCHEME 11 The photosynthetic machinery (PS‐I and PS‐II photosystems I and II)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 3
chloroplast mEP is a starting block in the biosynthesis of terpenes through C
5 isoprene units (isopentenyl diphosphate
(IPP) and dimethylallyl diphosphate (DmAPP)) especially those in C
10 C
20 and C
40 terpenes 3‐PGA is a precursor of
serine and other amino acids while phosphoenolpyruvate (PEP) the precursor of PA is also an intermediate toward the previously mentioned shikimic acid Lastly PA is not only a precursor of the fundamental ldquoC
2rdquo acetyl coenzyme A
(AcCoA) unit but also an intermediate toward aliphatic amino acids and mEP
AcCoA is the building block of fatty acids polyketides and mevalonic acid (mVA) a cytosolic precursor of the C
5 isoprene units for the biosynthesis of terpenes in the
C15
and C30
series (mind it is different from the mEP pathway in product and in cell location) Finally AcCoA enters the citric acid or Krebs cycle which leads to several precursors of amino acids These are oxaloacetic acid precursor of aspartic acid through transamination (thus toward lysine as a nitrogenated C
5N linear unit and methishy
onine as a methyl supplier) and 2‐oxoglutaric acid preshycursor of glutamic acid (and subsequent derivatives such as ornithine as a nitrogenated C
4N linear unit) All these
amino acids are key precursors in the biosynthesis of many alkaloids
114 Reactions Involved in the Construction of Secondary Metabolites
most reactions occurring in the living cells are performed by specialized enzymes which have been classified in an intershynational nomenclature defined by an enzyme commission (EC) number There are six classes of enzymes depending on the biochemical reaction they catalyze EC‐1 oxidoreducshytases (catalyzing oxidoreduction reactions) EC‐2 transfershyases (catalyzing the transfer of functional groups) EC‐3 hydrolases (catalyzing hydrolysis) EC‐4 lyases (breaking bonds through another process than hydrolysis or oxidation leading to a new double bond or a new cycle) EC‐5 isomershyases (catalyzing the isomerization of a molecule) and EC‐6 ligases (forming a covalent bond between two molecules) many subclasses of these enzymes have been described depending on the type of atoms and functional groups involved in the reaction and if any on the cofactor used in this reaction For example several cofactors can be used by dehydrogenases like NAD(P)NAD(P)H FADFADH
2 or
FmNFmNH2 For a description of this classification the
reader can refer to specialized Internet websites like ExplorEnz [1] What is important to realize is that most enzymes are substrate specific and have been selected during
CHOHO
CO2H
O
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
HOOH
HO
HOH2CO
OHC
CH2OPO32ndash
OHHO
CO2H
HO
OH
OH
TYR
PHE
CO2H
NH2
TRP
CH2OPO32ndash
HO
HOH2COH
MEP
C10 C20 and C40 terpenes
CO2HHO
SER
GLY CYS
CO2HOPO3
2ndash
CH2OH
OHHO2C
MVA
C15 and C30 terpenes
Polyketides
Krebscycle
(citric acid)
VAL ALA ILE LEU
CO2H
O
HO2C
CO2HO
CO2H
ASP
LYS
GLU
Glucose-6-phosphate
Fructose-6-phosphate
Erythrose-4-phosphate
3-PGAL
OP2O63ndash OP2O6
3ndash
IPP DMAPP
3-PGA PEP PA
O SCoAAcCoA
3x
Cytosol
Cytosol
Chloroplasts
Chloroplasts
Glycolysis
C2
C5
Shikimic acid Oxaloaceticacid
2-Oxoglutaricacid
MET
THR
PRO
ARGORN
Alkaloids Alkaloids Alkaloids Alkaloids
Aminoacids
Peptidesproteins
Phenolics
C1
C5N C4N
ldquoCH3rdquo
(Indole)C2NC6C3C6C2N C6C1
Pentosephosphatepathway
SCHEME 12 The building block chart involving glycolysis and the Krebs cycle
4 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
evolution to perform specific transformations making natural products with often and yet unknown functions
Secondary metabolites arise from specific biosynthetic pathways which use the previously defined building blocks The bunch of organic reactions involved in these biosynshytheses allows the construction of natural product frameshyworks which are finally diversified through ldquodecorationrdquo steps (Scheme 13) It is not the purpose of this introductive chapter to describe in detail all biosynthetic pathways and the reader can refer to excellent books and articles which have been published elsewhere [2 3]
The reactions involved in the construction of natural product skeletons will be described later for representative classes of compounds The identification of the building block footprint in the natural product skeleton will be emphasized as much as possible sometimes referring to biogenetic speculations [4] After the framework construction the decoration steps will involve as diverse reactions as aliphatic CH oxidations (eg involving a cytochrome P
450 oxygenase) occasionally triggering a
rearrangement heteroatom alkylations (eg methylation by S‐adenosylmethionine) or allylation (by DmAPP) esterifications heteroatom or C‐glycosylations (leading to heterosides) radical couplings (especially for phenols) alcohol oxidations or ketone reductions amineketone transaminations alkene dihydroxylations or epoxidations oxidative halogenations BaeyerndashVilliger oxidations and further oxygenation steps At the end of the biosynthesis such transformations may totally hide the primary building block origin of natural products
115 Secondary Metabolisms
1151 Polyketides Polyketides (or polyacetates) are issued from the oligomerization of C
2 acetate units performed
by polyketide synthases (PKS) and leading to (C2)
n linear
intermediates [5 6] If the (C2)
n intermediates arise from
successive Claisen reactions performed by ketosynthase
domains (KS in nonreducing PKS) a highly reactive poly‐ β‐ketoacyl intermediate H(CH
2CO)
nOH is formed
leading to phenolic and aromatic products through further intramolecular Claisen condensations Furthermore highly reducing PKSs are made of specialized enzymatic subunits working in line or iteratively to functionalize each C
2 linker
bond as CH(OH)CH2 (by ketoreductases (KR)) then as
HCCH (by dehydratases (DH)) and as CH2CH
2 (by enoyl
reductases (ER)) leading to a high degree of functionalization of the final product (Fig 12) By these iterative sequences highly reduced polyketides which can be either linear macrocyclized or polycyclized depending on the reactivity of the biosynthetic intermediates can be formed [7] With the same logic fatty acids are also biosynthesized by fatty acid synthases
moreover the PKS enzyme can be hybridized with nonshyribosomal peptide synthetase (NRPS) domains (see also ldquoNRPS metabolites and peptidesrdquo in the ldquoAlkaloidsrdquo secshytion) leading to the acylation of an amino acid by the (C
2)
n
acyl intermediate As previously the functionalization of the acyl chain depends on the PKS enzyme and the PKSNRPS products are also extremely diversified (eg hirsutellone B Fig 12) [8]
1152 Terpenes Terpenes are derived from the oligoshymerization of the C
5 isoprene units DmAPP and IPP Both
precursors are prompt to generate either an allylic cation (the diphosphate is a good leaving group) or a tertiary carbocation respectively which makes the IPP easy to react with DmAPP (Scheme 14) This reaction happens in the active site of a terpene synthase which activates the deparshyture of the diphosphate group from DmAPP thanks to Lewis acid activation (a metal like mg2+ mn2+ or Co2+ is present in the enzyme active site [9]) This leads to geranyl (C
10
monoterpene precursor) or farnesyl (C15
sesquiterpene precursor) diphosphate depending on the location of the enzyme (chloroplast for the mEP pathway or cytosol for the mVA pathway) Geranylgeranyl (C
20 diterpene) and
Constructionreactions
Decoration reaction(functionalization)Building
blocksNatural product
frameworksNatural products
HH
OH OAcH
HO O OH
OHO
OBz
P450
P450
P450
P450
P450 P450 Oxidativeauxiliaryenzymes
Taxadiene
OP2O63ndash
OP2O63ndash
3times
(a)
(b)
10-Deacetylbaccatin III
DMAPP
IPPand electrophilic
cyclizations
SCHEME 13 (a) From building blocks to natural products and (b) the example of 10‐deacetylbaccatin III
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 5
farnesylfarnesyl (C30
triterpene precursor) diphosphates can also be obtained by further additions of IPP leading to longer linear intermediates
The cyclization of linear precursors is achieved by speshycialized cyclases which generate a poorly functionalized natural product framework [10 11] Auxiliary enzymes such as oxygenases then increase the complexity and the diversity
of compounds by further functionalization (Scheme 13b) [12] A high degree of oxidation can be observed in comshypounds like thapsigargin paclitaxel or bilobalide (Fig 13) The biosynthesis of this last compound for example involves a high oxygenation pattern two Wagnerndashmeerwein rearrangements and several oxidative cleavages leading to the loss of five carbons The resulting natural products can
KS
S-ACP
O O
KR
S-ACP
OH O
S-MAT
O
YesNo
R
R
R
YesNoDH
S-ACP
OR
YesNoER
S-ACP
OR
YesNoTE
OH
OR
New cycle
New cycleor
release
New cycleor
release
New cycleor
release
Release
R in
crem
ente
d by
two
carb
ons
HO
O
S-ACP
O
Claisen condensation (ndashCO2)
reduction (NADPH)
dehydration (ndashH2O)
+
reduction (NADPH)
hydrolysis (H2O)
O O NH
OH
OH
H H
H H
Hirsutellone B(mixed PKSNRPS
product)
OO
O
OHOH
HO OH
Norsolorinic acid
O
O
O
O CHO
OH
HH
H
H
HH
OHHemibrevetoxin B
OOHO
OHO
HOHO OH
Erythronolide A
OH
OH
HO
Panaxytriol
From ahexanoylstartingblock
H
H
Fromtyrosine
H
1C lost fromdecarboxylation
FIGURE 12 Chemical logic of polyketide construction leading to variable functionalization of the elongated acyl chain and examples of resulting chemical diversity ACP acyl carrier protein DH dehydratase ER enoyl reductase KR ketoreductase KS ketosynthase mAT malonyl acyl transferase TE thioesterase
DMAPPOP2O6
3ndashOP2O6
3ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
Geranyl diphosphate (GPP)2 times IPP
H H
Geranylgeranyl diphosphate (GGPP)
Examplesverticillane
casbanetaxane
phorbol
Exampleslabdane
pimaranekauraneabietane
aphidicolanegibberellane
IPP
SCHEME 14 Early assembly of C5 units in terpene biosynthesis leading to diterpenes (C
20)
6 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
thus be extremely modified with structures whose biogenetic origin is far from being obvious at first sight and cannot be determined without further experiments such as isotopic labeling
1153 Flavonoids Resveratrols Gallic Acids and Further Polyphenolics We have previously discussed the polyketide origin of some phenolic compounds based on the (C
2)
n motif Other polyphenols like gallic acids are directly
derived by the aromatization of shikimic acid (C6C
1 building
block Scheme 12) [13] The C6C
3 building blocks are availshy
able from the conversion of phenylalanine and tyrosine into cinnamic and p‐coumaric acids respectively and then by further hydroxylation steps (Scheme 15) These can dimerize into lignans (eg podophyllotoxin) [14 15] through radical processes or converted to low molecular weight compounds like eugenol coumarins or vanillin [16] The coenzyme A thioesters of these C
6C
3 acids can be used as initiator units
by specialized ketosynthases for an elongation by two acetyl units leading to aromatic polyketides like styrylpyrones or diarylheptanoids (eg curcumin) [17] Important comshypounds from this metabolism are flavonoids (C
6C
3C
6) [18]
and stilbenoids (C6C
2C
6) (a decarboxylation occurs during
the aryl cyclization) [19] which are synthesized by chalcone synthase and stilbene synthase respectively Flavonoids (eg catechin) and stilbenes (eg resveratrol) are present in large amounts in fruits and vegetables and may exert their radical scavenging properties in vivo
1154 Alkaloids Alkaloids are nitrogen‐containing compounds The nitrogen(s) can be involved in an amine function conferring basicity to the natural product (like ldquoalkalirdquo) or in less or nonbasic functions such as an amide a nitrile an isonitrile or an ammonium salt (quaternary amines) For amines protonation often occurs at physiological pH and may condition their biological activity In many cases the nitrogen is biogenetically derived from an amino acid We will thus discuss alkaloids according to their amino acid origin
Alkaloids Derived from the Krebs Cycle (Lysine and Ornithine Derived) As shown previously (Scheme 12) the Krebs cycle is a crucial metabolic process which leads to α‐ketoacids (oxaloacetic and 2‐oxoglutaric acids) Their enzymatic transamination affords the two amino acidsmdashaspartic acid and glutamic acidmdashwhich are the direct biosynthetic precursors of amino acids lysine and ornithine respectively These in turn produce cadaverine a ldquoC
5Nrdquo unit
and putrescine a ldquoC4Nrdquo unit which are major components
for the biosynthesis of important alkaloids as will be discussed later (Scheme 16) Additionally ornithine is a precursor of arginine another important amino acid
ornithine‐derived alkaloids (incorporating the c4n
unit) Putrescine is derived from the decarboxylation of ornithine and is a precursor of linear polyamines like spermshyine After enzymatic methylation of one amine of putrescine
C10 C15
C15
C10
C10
C30
CO2H
Chrysanthemic acid
OH
Menthol
O
HO
H
HOGlc
MeO2CLoganin (iridoid)
H
H
H
HH
OO
OParthenolide
O
O
OHOHO
OAcHO
OnC3H7
O
O
O
nC7H15
Thapsigargin
O
O
OO
H
H
O
Cleavage
H
H
Artemisinin
C20 C40
O
O
OO
OO
OHOHH
Bilobalide(highly rearrangedand missing 5 C)
CleavageH
Highoxidativecleavages
HO OAcH
AcO O OH
OOOBz
O
Ph
BzNH
OHFrom PHE Paclitaxel
C25O
H
HO
OHCH
OH Ophiobolin A
H
HO
H
H
HHCholesterol
(missing 3 C WM)
H
H H H
H
H
Hopene
β-Carotene
C30 - 3
C15
C20 - 5WM
WM
FIGURE 13 Chemical diversity in the terpene series (Wm Wagnerndashmeerwein shifts lost carbons bold bonds are remnant of primary building blocks)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 7
in the presence of S‐adenosylmethionine transamination of the other affords γ‐(N‐methylamino)aldehyde [20] The resulting cyclic iminium is a key intermediate in the formation of many medicinally important alkaloids such as
the plant‐derived compounds cocaine atropine or the calysshytegines [21 22] Indeed this iminium is a mannich acceptor which can react with various nucleophiles the first of those being the carbanion of acetyl‐CoA Thus after a stepwise
CO2H
RCinnamic acid (R=H)p-Coumaric acid (R=OH)
RO
PHETYR
OH
[H] [O]
lignans
OH
O
O
O
O
OMeMeO
OMe
C mdash C andor C mdash Oradical couplings
After E Zisomerization
for example Podophyllotoxin
O ORO
Coumarins(eg scopoletin)
[O]
Prenylationcyclizationcleavage[O]
O OO
Furocoumarins(eg psoralen)
RO
nAcetyl-CoA
thencyclization
n = 2 styrylpyrones
O O
OMe
n = 3 avonoids(from chalcone synthase)
C6C3
H
H H
From AcCoA
O
n = 3 stilbenoids(from stilbene synthase)
HO
OH
OHFrom AcCoA From AcCoA(ndashCO2)
Resveratrol
HO
OH
OH
OH
OH
H
Catechin
MeO Yangonin
From AcCoA
SCHEME 15 The phenylpropanoid biosynthetic pathways
LYS
CO2
Cadaverine
O NH
H2O
NH
Piperideineiminium
Pelletierine
AcAcCoA
CO2
NH
O
N
O
Pseudopelletierine
NH
Ph
OH
Ph
O
Lobeline
NH
δ+
δndash
N
H
H
N
H
OH
Lupinine
N
H N
H
(+)-Sparteine
N
NH
O (ndash)-Cytisine
NH
CO2H
Pipecolic acid
N
OHHO
OH
HO
H
Castanos permine
ORN
CO2
NH2 NH2 NH2 NH2
NH2
NH2
NH2
PutrescineO
NHMe
N-Methylpyrrolinium
2 timesAcCoA
NMe
ARG
n = 1 spermidinen = 2 homospermidine
NMe
O
HygrineCO2
CO2
MeN
O
[O]
TropinoneCO2
HNHO
OHOH
OH
Calystegine B2
MeN
OAtropine
O
Ph
HO
NMe
NMe
NMe
O
Cuscohygrine
HN
HN
NH2
( )n
O
n = 2
N
HHO
Retronecine
HH
H
H
H
HO
H H
H
SCHEME 16 Lysine‐ and ornithine‐derived alkaloid biosynthetic pathways (mind the structural similarities)
8 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
elongation by two AcCoA units either decarboxylation can occur leading to the acetonylpyrrolidine hygrine or a secshyond mannich reaction by the intramolecular attack of the acetoacetate anion onto an oxidation‐derived pyrrolinium leading to the tropane skeleton (tropinone) The acetoacetyl‐CoA intermediate can also react intermolecularly with another pyrrolinium cation leading to cuscohygrine after decarboxylation Finally the pyrrolizidine alkaloids [23] are derived from homospermidine which when submitted to terminal oxidative deamination leads to the bicyclic skelshyeton of retronecine and further Senecio alkaloids We can mention herein that ornithine is a biosynthetic precursor of arginine bearing a guanidine function which is an intermediate toward the toxic compounds tetrodotoxin and saxitoxin (not shown)
lysine‐derived alkaloids (incorporating the c5n
unit) From lysine to piperidine alkaloids the biosynthetic steps parallel the one previously described from ornithine Indeed the oxidative deamination of cadaverine affords a δ‐amino aldehyde which cyclizes through imine formation into piperideine Protonation results in a mannich acceptor which is able to react with various nucleophiles such as β‐ketothioester anions The first product of these reactions is pelletierine which can further react through an intramolecular mannich
reaction leading to pseudopelletierine Quinolizidines [24] can also be formed first from the mannich reaction of the piperideine acceptor with the corresponding enamine nucleoshyphile and then after additional transformation steps leading for example to lupinine sparteine or cytisine
Indolizidine alkaloids [15] such as castanospermine and swainsonine are formed from pipecolic acid an amino acid derived from lysine which can be elongated by malonyl‐CoA followed by ring closure When protonated these alkaloids are oxonium mimics strongly inhibiting glycosidases
Tyrosine‐ and Phenylalanine‐Derived Alkaloids Tyrosine and phenylalanine amino acids are bearing the phenylshyethylamine moiety of many medicinally relevant alkaloids Further hydroxylations on the aromatic carbocycle or on the aliphatic part can be observed methylations can occur on phenolic oxygens and on the amine leading to catecholamines (adrenaline noradrenaline dopamine) Arylethylamines are also usual to react with endogenous aldehydes through PictetndashSpengler reactions [25] leading to important biosynthetic intermediates (Scheme 17) like
bull Reticuline from the reaction with 4‐hydroxyphenylacetshyaldehyde toward benzyltetrahydroisoquinoline alkaloids
NH2
Phenylethylamines
RONH TetrahydroisoquinolinesRO
RʹCHO
Rʹ
NH
(CH2)n
MeO
HO
Benzyltetrahydroisoquinoline(n = 1) for example reticuline
orPhenethyltetrahydroisoquinoline
(n = 2) eg autumnaline
IntermolecularCmdashO phenol
couplings
Curarealkaloids
IntramolecularC mdash C phenol
couplings
ONMe
HHO
H
HO
Morphine
NMe
MeO
HO
MeOOH
Isoboldine
O
O
OMe
NO2
CO2H
Aristolochic acid
OMe
O
NHAcMeO
MeOMeO
Colchicine
N
O
O
OMe
OMeBerberine
IntramolecularCmdash C coupling
and furtheroxidative events
Rʹ derivedfrom PHE
Pictet-Spenglerreaction
TYR
HO
HO CHO
HNHO
HO
OH
OMeO
OH
NMe
[O]
GalantamineNorbelladine
Rʹ derived fromsecologanin
NAc
HO
HO
O
CO2MeH
H
H
OHIpecosideaglycone
Ipecac alkaloids
1ClostCmdash C cleavage
and ring extension
H
ROH
1C lost
Cleavage
SCHEME 17 Tyrosine‐derived alkaloid biosynthetic pathways (double head arrows figure bond cleavages during biosynthetic processes)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 9
morphine berberine tubocurarine isoboldine or the highly modified aristolochic acid [26 27]
bull Automnaline from the reaction with 3‐(4‐hydroxyphenyl)propanal toward phenylethyltetrahydroisoquinoline alkaloids colchicine cephalotaxine or schelhammerishycine [28 29]
bull Ipecoside from the reaction of dopamine with secoloshyganin toward terpene tetrahydroisoquinoline alkaloids ipecoside or emetine
Lastly norbelladine (top of Scheme 17) is issued from the reductive amination of 34‐dihydroxybenzaldehyde (derived from phenylalanine) with tyramine (derived from tyrosine) and constitutes a biosynthetic node leading to Amaryllidaceae alkaloids such as galantamine crinine or lycorine depending on the topology of phenolic couplings In all these biosynthetic routes radical phenolic couplings are key reactions for CC and CO bond formations and rearrangements [30 31]
Tryptophan‐Derived Indole and Indole Monoterpene Alkaloids As for alkaloids derived from tyrosine and phenylalanine those derived from tryptophan are formed after decarboxylation of the amino acid (into tryptamine) and possible hydroxylation of the aromatic carbocycle (eg serotonin) and N‐methylation (eg psilocin) As previously tryptamine can also react through PictetndashSpengler reactions to form tetrahydro‐β‐carbolines which can be aromatized for example into harmine (Scheme 18) [16]
When the aldehyde partner of the PictetndashSpengler reacshytion with tryptamine is the terpene secologanin strictosidine is formed as an entry toward the vast monoterpene indole alkaloids [32 33] Hydrolysis of the glucosidic part releases the strictosidine aglycone bearing an aldehyde while iminshyium formation and further cyclization and reduction can lead to ajmalicine (from oxocyclization) or yohimbine (from carshybocyclization) These alkaloids are referred to as from the Corynanthe type with the monoterpene carbon skeleton unmodified Although it misses one carbon and has a very
RO
Carbonylcompounds
TRPNH
NH2
Indolethylamines(eg serotonine)
RONH
NH
Rʹ NH
N
MeOPictet-Spenglerreaction for example Harmineβ-Carbolines
NH
NHH
O
MeO2C
MeO2C
OH
H
H
Rʹ derived from secologanin
Strictosidine aglycone
NH
N
OH
H
H
Ajmalicine
N
N
O
O
H
H
H
Strychnine (C lost)Corynanthe type
Aspidosperma type Iboga type
N
N
OH
OMe
Quinine (C lost)
N
NH CO2Me
Catharanthine
NH
N
CO2MeH
OAc
OH
Vindoline
Complextransformations
NH
NMe
HN
MeN
H
H
Chimonanthine
Cyclizationdimerization
Prenylationcyclization
NH
NMeHO2C
H
Lysergic acid
H
H
Ergot alkaloidsPyrroloindoles Indole monoterpene
1C lost
1C lostFrom AcCoA
SCHEME 18 Tryptophan‐derived alkaloid biosynthetic pathways (gray parts monoterpenic units)
10 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
different structure strychnine is related to the Corynanthe alkaloids incorporating two carbons from acetyl‐CoA Highly modified monoterpene skeletons are derived from the Corynanthe core through CC bond breaking and reorshyganization leading to Iboga‐type (eg catharanthine) and Aspidosperma‐type (eg vindoline) alkaloids The antishycancer drug vinblastine is a heterodimer resulting from the nucleophilic attack of vindoline on a mannich acceptor resulting from catharanthine found in madagascar perishywinkle (Catharanthus roseus) The heteroaromatic comshypounds ellipticine camptothecin and quinine are also derived from a Corynanthe‐type precursor although in this case the biosynthetic relationship may not be obvious due to deep modifications of the skeleton
Finally two important classes of compounds have to be mentioned since they have inspired many synthetic chemists The pyrroloindole alkaloids result from the cyclization of tryptamine as found in physostigmine (formed by a cationic mechanism after methylation in position 3 of the indole not shown) or in chimonanthine (presumably formed by a radical coupling mechanism Scheme 18) The ergot alkaloids are derived from the 33‐dimethylallylation on position 4 of the indole in trypshytophan whose further cyclization and oxidation processes afford the natural products (eg lysergic acid Scheme 18 and ergotamine) which have had important medical applications [34]
NRPS Metabolites and Peptides NRPS enzymes assemble amino acids including nonproteinogenic ones into oligopeptides The enzymes contain several modules and especially an adenylation domain (A) which specifically selects and activates the amino acid to be transferred as a thioester on the nearby peptidyl carrier protein (PCP) [2] A condensation module (C) then catalyzes the formation of the peptide bonds between the newly introduced amino acyl‐PCP (bearing a free amine) and the elongated peptidyl‐PCP thioester At the end of the elongation a cyclization can occur into cyclopeptides but the peptide can also be
transferred to auxiliary enzymes like methyltransferases glycosyltransferases or oxidases (vancomycins are typical products of such functionalizations) [35 36] The formation of heterocycles is also frequently encountered in this metabolism as in penicillins that are derived from the tripeptide α‐aminoadipoyl‐cysteinyl‐valine or telomestatin (Fig 14) [2]
Other Alkaloid Origins There are many other nitrogen sources involved in alkaloid biosyntheses for example nicotinic acid (originated from aspartic acid and intershymediate in nicotine and anabasine biosyntheses) and anthranilic acid (originated from tryptophan and intermedishyate toward acridines or aurachins) The amination reaction (eg through transamination of carbonyl compounds) is also a way to introduce nitrogens in natural products for example from fatty acids steroids (toward Solanum alkaloids or cyclopamines) or other terpenoids (aconitine and atisine have diterpene skeletons while Daphniphyllum alkaloids are triterpene derivatives) Finally nucleic acids can also be precursors of alkaloids like the well‐known caffeine
12 FROM BIOSYNTHESIS TO TOTAL SYNTHESIS STRATEGIES TOWARD THE NATURAL PRODUCT CHEMICAL SPACE
121 The Chemical Space of Natural Products
Natural products occupy an important place in human com munities as demonstrated by their vast use from ancient times to nowadays like dyes fibers oils pershyfumes agrochemicals or drugs Broadly both primary and secondary metabolites could be classified as natural products while the latter as discussed previously are usushyally regarded as the ldquonatural productsrdquo owing to their comshyplexity and diversity arising from a variety of biosynthetic pathways The structural chemical diversity found among
N
S
CO2HO
HNPhH2C
O
Penicillin G
OH
HN
HN
O
O
ONH2
O
OHO
NHMe
OCl
O
NH
NH
NH
O
ClHO
O
HN
H
H
HOOH
OHHO2C
Vancomycin aglycone
ON
NO
O
NN
O
O
N
N O
Me
S
N N
OMeH
Telomestatin
L-AAA-L-CYS-D-VAL
Enzymes
FIGURE 14 Structural diversity of nonribosomal peptide compounds (AAA α‐aminoadipic acid)
FROm BIOSyNTHESIS TO TOTAL SyNTHESIS STRATEGIES TOWARD THE NATuRAL PRODuCT CHEmICAL SPACE 11
all living organisms defining the chemical space of natural products [37] is the consequence of their evolution occurshyring as an adaptation of organisms to their environment It is commonly believed that secondary metabolites are proshyduced as messengers by living organisms or as weapons against enemies and thus they should have certain biological activities in a medicinal point of view [38] Indeed natural products are regarded as one of the main sources of medicines (Fig 15) From the traditional medicinal extracts to every single bioactive molecule the methods of extraction purification identification and biological investigation of natural products have been well established Their complex structures and interesting properties have attracted synthetic chemists to accomplish their total syntheses and that of medicinally relevant anashylogs sometimes in the industrial context [39] Thus tarshygeting the chemical space of natural products has never been more relevant than today Although the discovery of natural products demands time and labor‐consuming manipulations it is worth to notice that the knowledge on this chemical space is still continually growing while biological advances allow for discovering and undershystanding potential targets However increasing the chemical space of human‐made compounds based on natural products should benefit from transdisciplinary colshylaborations such as the use of coupled biosynthetic and chemical synthetic methods to design original ldquounnatural natural productsrdquo [40]
122 The Biosynthetic Pathways as an Inspiration for Synthetic Challenges
1221 Precursor‐Directed Biosyntheses and Mutashysynthetic Strategies to Increase the Chemical Space of Natural Products As the genetics and biochemistry of natural product biosynthesis are better understood novel biosynthetic techniques have been developed to study and generate new diversity in natural product analogs (Scheme 19) Precursor‐directed biosynthesis (PDB) is considered as the earliest example of combining chemical and biological methods for the generation of complex natural product analogs [41 42] This approach compared with the biosynthetic pathway of wild‐type metabolites (Scheme 19a) involves the feeding of analogs of the natural biosynthetic building blocks to the living organisms (Scheme 19b) usually bacteria or fungi which incorporate the modified precursors into the biosynthesized compound mutasynthesis also termed as mutational biosynthesis (mBS) involves the inactivation of a key step of the bioshysynthesis in a mutant microorganism (Scheme 19c) which can then be fed by various modified or advanced building blocks (mutasynthons Scheme 19d) [43] These mutasshyynthons could not be incorporated by the wild type due to specificities of the enzymatic machinery Build up on PDB mBS eliminates the natural biosynthetic intermediate thus generating a less complex mixture of metabolites and making the purification or yield of target products better
O NH
OHO
O
OO
O
O
OH
HOO
O O
OH
HOO
NH ONH2
O
ONH
NH
O
OCl Cl
O
O
OH
HN
HN
HN
HN
OO
HO
HN
OH
OHOH
O
O
HO
HO
OHO
O
OHH2N
O
OH3C
H
O
H
CH3
CH3
H
O O
NO
H
O
HO
O
NO
O
NH
HN
OHO
HONH
ON
H
HO
O
H2N
O OH
ONH
OH
ON OHO
OH
HO OS OH
OOOH
OHO
O
ON
OO
O
HO
O
O OH
OO
Micafunginantifungal
Rapamycinimmunosuppressive
Galantamineanti-Alzheimer
Taxolanticancer
Artemisininantimalarial
Vancomycinantibiotic
FIGURE 15 Some famous natural products currently used as drugs
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product
x CoNtENtS
738 Diverse Sesquiterpenoids 27674 Conclusion 276References 276
8 Diterpenes 279Louis Barriault
81 Introduction 27982 Biosynthesis of Diterpenes Based on Cationic Cyclizations
12‐Shifts and transannular Processes 27983 Pericyclic Reactions and their application in the Synthesis
of Selected Diterpenoids 284831 Dielsndashalder Reaction and Its application in the total
Synthesis of Diterpenes 284832 Cascade Pericyclic Reactions and their application in the total
Synthesis of Diterpenes 29184 Conclusion 293
References 294
9 Higher Terpenes and Steroids 296Kazuaki Ishihara
91 Introduction 29692 Biosynthesis 29693 Cascade Polyene Cyclizations 303
931 Diastereoselective Polyene Cyclizations 303932 ldquoChiral proton (H+)rdquo‐Induced Polyene Cyclizations 304933 ldquoChiral Metal Ionrdquo‐Induced Polyene Cyclizations 308934 ldquoChiral Halonium Ion (X+)rdquo‐Induced Polyene Cyclizations 313935 ldquoChiral Carbocationrdquo‐Induced Polyene Cyclizations 319936 Stereoselective Cyclizations of Homo(polyprenyl)arene
analogs 31994 Biomimetic total Synthesis of terpenes and Steroids through
Polyene Cyclization 31995 Conclusion 328
References 328
SECTION III SHIKIMIC ACID BIOSYNTHETIC PATHWAY 331
10 Lignans Lignins and Resveratrols 333Yu Peng
101 Biosynthesis 3331011 Primary Metabolism of Shikimic acid and aromatic
amino acids 3331012 Lignans and Lignin 335
102 auxiliary‐assisted C(sp3)ndashH arylation Reactions in organic Synthesis 336
103 FriedelndashCrafts Reactions in organic Synthesis 344104 total Synthesis of Lignans by C(sp3)H arylation Reactions 353105 total Synthesis of Lignans and Polymeric Resveratrol by
FriedelndashCrafts Reactions 357106 Conclusion 375References 375
CoNtENtS xi
SECTION IV MIXED BIOSYNTHETIC PATHWAYSndash THE STORY OF ALKALOIDS 381
11 Ornithine and Lysine Alkaloids 383Sebastian Brauch Wouter S Veldmate and Floris P J T Rutjes
111 Biosynthesis of l‐ornithine and l‐Lysine alkaloids 3831111 Biosynthetic Formation of alkaloids
Derived from l‐ornithine 3831112 Biosynthetic Formation of alkaloids
Derived from l‐Lysine 388112 the asymmetric Mannich Reaction in organic Synthesis 392
1121 Chiral amines as Catalysts in asymmetric Mannich Reactions 3941122 Chiral Broslashnsted Bases as Catalysts in asymmetric
Mannich Reactions 3981123 Chiral Broslashnsted acids as Catalysts in asymmetric
Mannich Reactions 4041124 organometallic Catalysts in asymmetric Mannich Reactions 4081125 Biocatalytic asymmetric Mannich Reactions 413
113 Mannich and Related Reactions in the total Synthesis of l‐Lysine‐ and l‐ornithine‐Derived alkaloids 414
114 Conclusion 426References 427
12 Tyrosine Alkaloids 431Uwe Rinner and Mario Waser
121 Introduction 431122 Biosynthesis of tyrosine‐Derived alkaloids 431
1221 Phenylethylamines 4311222 Simple tetrahydroisoquinoline alkaloids 4331223 Modified Benzyltetrahydroisoquinoline alkaloids 4331224 Phenethylisoquinoline alkaloids 4361225 amaryllidaceae alkaloids 4381226 Biosynthetic overview of tyrosine‐Derived alkaloids 442
123 arylndasharyl Coupling Reactions 4421231 Copper‐Mediated arylndasharyl Bond Forming Reactions 4431232 Nickel‐Mediated arylndasharyl Bond Forming Reactions 4461233 Palladium‐Mediated arylndasharyl Bond Forming Reactions 4471234 transition Metal‐Catalyzed Couplings of Nonactivated
aryl Compounds 450124 Synthesis of tyrosine‐Derived alkaloids 456
1241 Synthesis of Modified Benzyltetrahydroisoquinoline alkaloids 4561242 Synthesis of Phenethylisoquinoline alkaloids 4601243 Synthesis of amaryllidaceae alkaloids 462
125 Conclusion 468References 469
13 Histidine and Histidine‐Like Alkaloids 473Ian S Young
131 Introduction 473132 Biosynthesis 473133 atom Economy and Protecting‐Group‐Free Chemistry 480
xii CoNtENtS
134 Challenging the Boundaries of Synthesis PIas 488135 Conclusion 497References 499
14 Anthranilic AcidndashTryptophan Alkaloids 502Zhen‐Yu Tang
141 Biosynthesis 502142 Divergent SynthesisndashCollective total Synthesis 508143 Collective total Synthesis of tryptophan‐Derived alkaloids 510
1431 Monoterpene Indole alkaloids 5101432 Bisindole alkaloids 512
References 517
15 Future Directions of Modern Organic Synthesis 519Jakob Pletz and Rolf Breinbauer
151 Introduction 519152 Enzymes in organic Synthesis Merging total
Synthesis with Biosynthesis 520153 Engineered Biosynthesis 526154 Diversity‐oriented Synthesis Biology‐oriented Synthesis
and Diverted total Synthesis 5331541 Diversity‐oriented Synthesis 5351542 Biology‐oriented Synthesis 5361543 Diverted total Synthesis 539
155 Conclusion 541References 545
INDEX 548
Elissavet E Anagnostaki Department of Chemistry Laboratory of Organic Chemistry Aristotle University of Thessaloniki Thessaloniki Greece and Research and Development Department Pharmathen SA Thessaloniki Greece
Stellios Arseniyadis School of Biological and Chemical Sciences Queen Mary University of London London United Kingdom
Louis Barriault Department of Chemistry University of Ottawa Ottawa Ontario Canada
Erica Benedetti Laboratoire de Chimie et Biochimie et Pharmacologiques et Toxicologiques CNRS-Universiteacute Paris Descartes Faculteacute des Sciences Fondamentales et Biomeacutedicales Paris France
Sebastian Brauch Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
Rolf Breinbauer Institute of Organic Chemistry Technische Universitaumlt Graz Graz Austria
Cyril Bressy Aix Marseille Universiteacute Centrale Marseille CNRS Marseille France
Martin Cordes Institute for Organic Chemistry and Center of Biomolecular Drug Research (BMWZ) Leibniz Universitaumlt Hannover Hannover Germany and Helmholtz Center for Infection Research (HZI) Hannover Germany
Paul E Floreancig Department of Chemistry Chevron Science Center University of Pittsburgh Pittsburgh PA USA
Tobias A M Gulder Department of Chemistry and Center for Integrated Protein Science Munich (CIPSM) Biosystems Chemistry Technische Universitaumlt Muumlnchen Munich Germany
Trond Vidar Hansen School of Pharmacy University of Oslo Oslo Norway
Kazuaki Ishihara Department of Biotechnology Graduate School of Engineering Nagoya University Nagoya Japan
Markus Kalesse Institute for Organic Chemistry and Center of Biomolecular Drug Research (BMWZ) Leibniz Universitaumlt Hannover Hannover Germany and Helmholtz Center for Infection Research (HZI) Hannover Germany
Xu‐Wen Li Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
Bastien Nay Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France
Yu Peng State Key Laboratory of Applied Organic Chemistry Lanzhou University Lanzhou China
Jakob Pletz Institute of Organic Chemistry Technische Universitaumlt Graz Graz Austria
Uwe Rinner Institute of Organic Chemistry Johannes Kepler University Linz Linz Austria and Department of Chemistry College of Science Sultan Qaboos University Muscat Oman
Floris P J T Rutjes Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
LIST oF CoNTRIBUToRS
xiv LIST OF CONTRIBUTORS
Franccediloise Schaefers Department of Chemistry and Center for Integrated Protein Science Munich (CIPSM) Biosystems Chemistry Technische Universitaumlt Muumlnchen Munich Germany
Michael Smietana Institut des Biomoleacutecules Max Mousseron CNRS Universiteacute de Montpellier ENSCM France
Zhen‐Yu Tang Department of Pharmaceutical Engineering College of Chemistry and Chemical Engineering Central South University Changsha China
Wouter S Veldmate Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
Anders Vik School of Pharmacy University of Oslo Oslo Norway
Mario Waser Institute of Organic Chemistry Johannes Kepler University Linz Linz Austria
Youwei Xie Max‐Planck‐Institut fuumlr Kohlenforschung Muumllheim Germany
Ian S Young Bristol‐Myers Squibb Company Chemical Development New Brunswick NJ USA
Alexandros L Zografos Department of Chemistry Laboratory of Organic Chemistry Aristotle University of Thessaloniki Thessaloniki Greece
There is pleasure in the pathless woodsthere is rapture in the lonely shore
there is society where none intrudesby the deep sea and music in its roar
I love not Man the less but Nature moreLord Byron
Preface
The first time I came across with the idea of editing a book that merges selected chapters of biosynthesis and total synthesis was when I was teaching postgraduate courses of natural product synthesis at Aristotle University of Thessaloniki This period I realized that the best way to teach youngsters synthesis was to start from the very origin of inspiration nature and its tools biosynthesis
Over the last decades biosynthesis is filling our gaps of understanding the complex mechanisms of nature and pro-vides useful sources of inspiration not only in the way natural products can be synthesized but also by directing synthetic chemists in developing atom‐economical efficient synthetic methods Several are the examples that mimic biosynthetic guidelines from modern iterative alkylations and aldol reactions to CH oxidations that compile nowadays the modern toolbox of organic synthesis
The handed book is constructed in the logic of presenting the parallel development of biosynthesis and organic meth-odology and how these can be applied in efficient syntheses of natural products The book is divided into four sections each representing the four major biosynthetic pathways of natural products namely acetate mevalonate shikimate biosynthetic pathways and the mixed biosynthetic pathways
of alkaloids These sections are divided into chapters that represent selected classes of natural products for example lipids sesquiterpenoids lignans etc Each of these chapters is further divided into three distinct subchapters (a) biosyn-thesis (b) methodological section and (c) application of the described methodology in the total synthesis of the described family of natural products By this way the readers can be focused in the direct comparison between biosyn-theses and the developed methodologies to construct the crucial for each class of natural product carbon bonds Although the book as it develops is focused on presenting the power of biosynthesis and how this power can be applied in providing inspiration for the efficient synthesis of natural products it was not the authors will to present only biomi-metic total syntheses but rather to exploit the modern synthetic methodologies and recognize their disabilities for further improvement
Of course this book will not have been realized without the excellent work of renowned scientists worldwide working either in the field of biosynthesis or total synthesis who collected the existing knowledge on biosynthesis analyzed the existing modern methodologies and presented a bouquet of selected total syntheses Throughout our
xvi PrEfACE
endeavor to complete this book I learned many things from their expertise but I also realized that only with tight collab-orations you can build long‐lasting friendships I would like to thank them all once again for their trust and effort to complete this book We all hope that the current work will contribute to a better understanding of the current status of
organic chemistry and to the discovery of novel strategies and tactics for the synthesis of natural products
Alexandros L ZografosSeptember 2015
Thessaloniki Greece
From Biosynthesis to Total Synthesis Strategies and Tactics for Natural Products First Edition Edited by Alexandros L Zografoscopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
11 FROM PRIMARY TO SECONDARY METABOLISM THE KEY BUILDING BLOCKS
111 Definitions
The primary and secondary metabolisms are traditionally distinguished by their distribution and utility in the living organism network Primary metabolites include carbohyshydrates lipids nucleic acids and proteins (or their amino acid constituents) and are shared by all living organisms on Earth They are transformed by common pathways which are studied by biochemistry (Fig 11) Secondary metabo-lites are structurally diverse compounds usually produced by a limited number of organisms which synthesize them for a special purpose like defense or signaling through specific biosynthetic pathways They are studied by natural product chemistry This distinction is not always so obvious and some compounds can be studied in the context of both primary and secondary metabolisms This is especially true nowadays with the use of genetic and biomolecular tools which tend to make natural product sciences more and more integrative However an important point to remember is that the primary metabolism furnishes key building blocks to the secondary metabolism It would be difficult to describe in detail the full biosynthetic pathshyways in this section We tried to organize the discussion as a vade mecum synthetically gathering information from extremely useful sources which will be cited at the end of this chapter
112 Energy Supply and Carbon Storing at the Early Stage of Metabolisms
The sunlight is essential to life except in some part of the deep oceans It provides energy for plant photosynthesis that splits molecules of water into protons and electrons and releases O
2 (Scheme 11) A proton gradient inside the plant
chloroplasts then drags a transmembrane ATP synthase comshyplex that produces adenosine triphosphate (ATP) while elecshytrons released from water are transferred to the coenzyme reducer nicotinamide adenine dinucleotide phosphate hydride (NADPH) A major function of chloroplasts is to fix CO
2 as a combination to ribulose‐15‐bisphosphate (RuBP)
performed by RuBP carboxylase (rubisco) forming an instable ldquoC
6rdquo β‐ketoacid This is cleaved into two molecules
of 3‐phosphoglycerate (3‐PGA) which is then reduced into 3‐phosphoglyceraldehyde (3‐PGAL a ldquoC
3rdquo triose phosshy
phate) during the Calvin cycle This is one of the major metabolites in the biosynthesis of carbohydrates like glucose and a biochemical mean for storing and retaining carbon atoms in the living cells
113 Glucose as a Starting Material Toward Key Building Blocks of the Secondary Metabolism
Glucose‐6‐phosphate arises from the phosphorylation of glucose It is the starting material of glycolysis an important process of the primary metabolism which consists in eight enzymatic reactions leading to pyruvic acid (PA)
FROM BIOSYNTHESES TO TOTAL SYNTHESES AN INTRODUCTION
Bastien Nay1 and Xu‐Wen Li2
1
1 Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France2 Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
2 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
(Scheme 12) Important intermediates for the secondary metabolism are produced during glycolysis Glucose glucose‐6‐phosphate and fructose‐6‐phosphate can be converted to other hexoses and pentoses that can be oligoshymerized and enter in the composition of heterosides Additionally fructose‐6‐phosphate connects the pentose
phosphate pathway leading to erythrose‐4‐phosphate toward shikimic acid which is a key metabolite in the biosynthesis of aromatic amino acids (phenylalanine tyrosine or C
6C
3
units) and C6C
1 phenolic compounds The next important
intermediate in glycolysis is 3‐PGAL which can be redishyrected toward methylerythritol‐4‐phosphate (mEP) in the
Primary metabolism Secondary metabolism
The field ofbiochemistry
The field ofnatural product
chemistry
Essential toliving organisms
Essential to theproducer organisms
under particularconditions
Nucleic acids (DNARNA) carbohydrates
lipids amino acidspeptides and proteins
Alkaloids terpenespolyketides
polyphenols andtheir heterosidic form
Biological effects(defense signaling)
Biosyntheticpathways
Main compoundclasses
FIGURE 11 Primary versus secondary metabolisms
PS-I
PS-IIendash
hν H2O H+ and O2
ATP synthase
NADP reductase
H+ (gradient)
H+ADP + Pi ATP
NADPHH+NADP+
H+
Thylakoidcompartment
Chloroplaststroma
Thylakoidmembrane
(a) Light dependent process
(b) Light independent process
CO2
RuBP
Rubisco
Unstable β-ketoacid3-PGA 13-diPGA
ATP
ADP3-PGAL
NADPHH+
NADP+
(Calvin cycle)
trioses tetroses pentoses hexoses (eg glucose) heptoses
Chloroplaststroma
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
CH2OPO32ndash CH2OPO3
2ndash CH2OPO32ndash
CH2OPO32ndash
OOH
HO
HOO
HO
HO2CCO2H
HO
CO2PO32ndash
HO
CHOHO
endash
Cytosol
SCHEME 11 The photosynthetic machinery (PS‐I and PS‐II photosystems I and II)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 3
chloroplast mEP is a starting block in the biosynthesis of terpenes through C
5 isoprene units (isopentenyl diphosphate
(IPP) and dimethylallyl diphosphate (DmAPP)) especially those in C
10 C
20 and C
40 terpenes 3‐PGA is a precursor of
serine and other amino acids while phosphoenolpyruvate (PEP) the precursor of PA is also an intermediate toward the previously mentioned shikimic acid Lastly PA is not only a precursor of the fundamental ldquoC
2rdquo acetyl coenzyme A
(AcCoA) unit but also an intermediate toward aliphatic amino acids and mEP
AcCoA is the building block of fatty acids polyketides and mevalonic acid (mVA) a cytosolic precursor of the C
5 isoprene units for the biosynthesis of terpenes in the
C15
and C30
series (mind it is different from the mEP pathway in product and in cell location) Finally AcCoA enters the citric acid or Krebs cycle which leads to several precursors of amino acids These are oxaloacetic acid precursor of aspartic acid through transamination (thus toward lysine as a nitrogenated C
5N linear unit and methishy
onine as a methyl supplier) and 2‐oxoglutaric acid preshycursor of glutamic acid (and subsequent derivatives such as ornithine as a nitrogenated C
4N linear unit) All these
amino acids are key precursors in the biosynthesis of many alkaloids
114 Reactions Involved in the Construction of Secondary Metabolites
most reactions occurring in the living cells are performed by specialized enzymes which have been classified in an intershynational nomenclature defined by an enzyme commission (EC) number There are six classes of enzymes depending on the biochemical reaction they catalyze EC‐1 oxidoreducshytases (catalyzing oxidoreduction reactions) EC‐2 transfershyases (catalyzing the transfer of functional groups) EC‐3 hydrolases (catalyzing hydrolysis) EC‐4 lyases (breaking bonds through another process than hydrolysis or oxidation leading to a new double bond or a new cycle) EC‐5 isomershyases (catalyzing the isomerization of a molecule) and EC‐6 ligases (forming a covalent bond between two molecules) many subclasses of these enzymes have been described depending on the type of atoms and functional groups involved in the reaction and if any on the cofactor used in this reaction For example several cofactors can be used by dehydrogenases like NAD(P)NAD(P)H FADFADH
2 or
FmNFmNH2 For a description of this classification the
reader can refer to specialized Internet websites like ExplorEnz [1] What is important to realize is that most enzymes are substrate specific and have been selected during
CHOHO
CO2H
O
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
HOOH
HO
HOH2CO
OHC
CH2OPO32ndash
OHHO
CO2H
HO
OH
OH
TYR
PHE
CO2H
NH2
TRP
CH2OPO32ndash
HO
HOH2COH
MEP
C10 C20 and C40 terpenes
CO2HHO
SER
GLY CYS
CO2HOPO3
2ndash
CH2OH
OHHO2C
MVA
C15 and C30 terpenes
Polyketides
Krebscycle
(citric acid)
VAL ALA ILE LEU
CO2H
O
HO2C
CO2HO
CO2H
ASP
LYS
GLU
Glucose-6-phosphate
Fructose-6-phosphate
Erythrose-4-phosphate
3-PGAL
OP2O63ndash OP2O6
3ndash
IPP DMAPP
3-PGA PEP PA
O SCoAAcCoA
3x
Cytosol
Cytosol
Chloroplasts
Chloroplasts
Glycolysis
C2
C5
Shikimic acid Oxaloaceticacid
2-Oxoglutaricacid
MET
THR
PRO
ARGORN
Alkaloids Alkaloids Alkaloids Alkaloids
Aminoacids
Peptidesproteins
Phenolics
C1
C5N C4N
ldquoCH3rdquo
(Indole)C2NC6C3C6C2N C6C1
Pentosephosphatepathway
SCHEME 12 The building block chart involving glycolysis and the Krebs cycle
4 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
evolution to perform specific transformations making natural products with often and yet unknown functions
Secondary metabolites arise from specific biosynthetic pathways which use the previously defined building blocks The bunch of organic reactions involved in these biosynshytheses allows the construction of natural product frameshyworks which are finally diversified through ldquodecorationrdquo steps (Scheme 13) It is not the purpose of this introductive chapter to describe in detail all biosynthetic pathways and the reader can refer to excellent books and articles which have been published elsewhere [2 3]
The reactions involved in the construction of natural product skeletons will be described later for representative classes of compounds The identification of the building block footprint in the natural product skeleton will be emphasized as much as possible sometimes referring to biogenetic speculations [4] After the framework construction the decoration steps will involve as diverse reactions as aliphatic CH oxidations (eg involving a cytochrome P
450 oxygenase) occasionally triggering a
rearrangement heteroatom alkylations (eg methylation by S‐adenosylmethionine) or allylation (by DmAPP) esterifications heteroatom or C‐glycosylations (leading to heterosides) radical couplings (especially for phenols) alcohol oxidations or ketone reductions amineketone transaminations alkene dihydroxylations or epoxidations oxidative halogenations BaeyerndashVilliger oxidations and further oxygenation steps At the end of the biosynthesis such transformations may totally hide the primary building block origin of natural products
115 Secondary Metabolisms
1151 Polyketides Polyketides (or polyacetates) are issued from the oligomerization of C
2 acetate units performed
by polyketide synthases (PKS) and leading to (C2)
n linear
intermediates [5 6] If the (C2)
n intermediates arise from
successive Claisen reactions performed by ketosynthase
domains (KS in nonreducing PKS) a highly reactive poly‐ β‐ketoacyl intermediate H(CH
2CO)
nOH is formed
leading to phenolic and aromatic products through further intramolecular Claisen condensations Furthermore highly reducing PKSs are made of specialized enzymatic subunits working in line or iteratively to functionalize each C
2 linker
bond as CH(OH)CH2 (by ketoreductases (KR)) then as
HCCH (by dehydratases (DH)) and as CH2CH
2 (by enoyl
reductases (ER)) leading to a high degree of functionalization of the final product (Fig 12) By these iterative sequences highly reduced polyketides which can be either linear macrocyclized or polycyclized depending on the reactivity of the biosynthetic intermediates can be formed [7] With the same logic fatty acids are also biosynthesized by fatty acid synthases
moreover the PKS enzyme can be hybridized with nonshyribosomal peptide synthetase (NRPS) domains (see also ldquoNRPS metabolites and peptidesrdquo in the ldquoAlkaloidsrdquo secshytion) leading to the acylation of an amino acid by the (C
2)
n
acyl intermediate As previously the functionalization of the acyl chain depends on the PKS enzyme and the PKSNRPS products are also extremely diversified (eg hirsutellone B Fig 12) [8]
1152 Terpenes Terpenes are derived from the oligoshymerization of the C
5 isoprene units DmAPP and IPP Both
precursors are prompt to generate either an allylic cation (the diphosphate is a good leaving group) or a tertiary carbocation respectively which makes the IPP easy to react with DmAPP (Scheme 14) This reaction happens in the active site of a terpene synthase which activates the deparshyture of the diphosphate group from DmAPP thanks to Lewis acid activation (a metal like mg2+ mn2+ or Co2+ is present in the enzyme active site [9]) This leads to geranyl (C
10
monoterpene precursor) or farnesyl (C15
sesquiterpene precursor) diphosphate depending on the location of the enzyme (chloroplast for the mEP pathway or cytosol for the mVA pathway) Geranylgeranyl (C
20 diterpene) and
Constructionreactions
Decoration reaction(functionalization)Building
blocksNatural product
frameworksNatural products
HH
OH OAcH
HO O OH
OHO
OBz
P450
P450
P450
P450
P450 P450 Oxidativeauxiliaryenzymes
Taxadiene
OP2O63ndash
OP2O63ndash
3times
(a)
(b)
10-Deacetylbaccatin III
DMAPP
IPPand electrophilic
cyclizations
SCHEME 13 (a) From building blocks to natural products and (b) the example of 10‐deacetylbaccatin III
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 5
farnesylfarnesyl (C30
triterpene precursor) diphosphates can also be obtained by further additions of IPP leading to longer linear intermediates
The cyclization of linear precursors is achieved by speshycialized cyclases which generate a poorly functionalized natural product framework [10 11] Auxiliary enzymes such as oxygenases then increase the complexity and the diversity
of compounds by further functionalization (Scheme 13b) [12] A high degree of oxidation can be observed in comshypounds like thapsigargin paclitaxel or bilobalide (Fig 13) The biosynthesis of this last compound for example involves a high oxygenation pattern two Wagnerndashmeerwein rearrangements and several oxidative cleavages leading to the loss of five carbons The resulting natural products can
KS
S-ACP
O O
KR
S-ACP
OH O
S-MAT
O
YesNo
R
R
R
YesNoDH
S-ACP
OR
YesNoER
S-ACP
OR
YesNoTE
OH
OR
New cycle
New cycleor
release
New cycleor
release
New cycleor
release
Release
R in
crem
ente
d by
two
carb
ons
HO
O
S-ACP
O
Claisen condensation (ndashCO2)
reduction (NADPH)
dehydration (ndashH2O)
+
reduction (NADPH)
hydrolysis (H2O)
O O NH
OH
OH
H H
H H
Hirsutellone B(mixed PKSNRPS
product)
OO
O
OHOH
HO OH
Norsolorinic acid
O
O
O
O CHO
OH
HH
H
H
HH
OHHemibrevetoxin B
OOHO
OHO
HOHO OH
Erythronolide A
OH
OH
HO
Panaxytriol
From ahexanoylstartingblock
H
H
Fromtyrosine
H
1C lost fromdecarboxylation
FIGURE 12 Chemical logic of polyketide construction leading to variable functionalization of the elongated acyl chain and examples of resulting chemical diversity ACP acyl carrier protein DH dehydratase ER enoyl reductase KR ketoreductase KS ketosynthase mAT malonyl acyl transferase TE thioesterase
DMAPPOP2O6
3ndashOP2O6
3ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
Geranyl diphosphate (GPP)2 times IPP
H H
Geranylgeranyl diphosphate (GGPP)
Examplesverticillane
casbanetaxane
phorbol
Exampleslabdane
pimaranekauraneabietane
aphidicolanegibberellane
IPP
SCHEME 14 Early assembly of C5 units in terpene biosynthesis leading to diterpenes (C
20)
6 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
thus be extremely modified with structures whose biogenetic origin is far from being obvious at first sight and cannot be determined without further experiments such as isotopic labeling
1153 Flavonoids Resveratrols Gallic Acids and Further Polyphenolics We have previously discussed the polyketide origin of some phenolic compounds based on the (C
2)
n motif Other polyphenols like gallic acids are directly
derived by the aromatization of shikimic acid (C6C
1 building
block Scheme 12) [13] The C6C
3 building blocks are availshy
able from the conversion of phenylalanine and tyrosine into cinnamic and p‐coumaric acids respectively and then by further hydroxylation steps (Scheme 15) These can dimerize into lignans (eg podophyllotoxin) [14 15] through radical processes or converted to low molecular weight compounds like eugenol coumarins or vanillin [16] The coenzyme A thioesters of these C
6C
3 acids can be used as initiator units
by specialized ketosynthases for an elongation by two acetyl units leading to aromatic polyketides like styrylpyrones or diarylheptanoids (eg curcumin) [17] Important comshypounds from this metabolism are flavonoids (C
6C
3C
6) [18]
and stilbenoids (C6C
2C
6) (a decarboxylation occurs during
the aryl cyclization) [19] which are synthesized by chalcone synthase and stilbene synthase respectively Flavonoids (eg catechin) and stilbenes (eg resveratrol) are present in large amounts in fruits and vegetables and may exert their radical scavenging properties in vivo
1154 Alkaloids Alkaloids are nitrogen‐containing compounds The nitrogen(s) can be involved in an amine function conferring basicity to the natural product (like ldquoalkalirdquo) or in less or nonbasic functions such as an amide a nitrile an isonitrile or an ammonium salt (quaternary amines) For amines protonation often occurs at physiological pH and may condition their biological activity In many cases the nitrogen is biogenetically derived from an amino acid We will thus discuss alkaloids according to their amino acid origin
Alkaloids Derived from the Krebs Cycle (Lysine and Ornithine Derived) As shown previously (Scheme 12) the Krebs cycle is a crucial metabolic process which leads to α‐ketoacids (oxaloacetic and 2‐oxoglutaric acids) Their enzymatic transamination affords the two amino acidsmdashaspartic acid and glutamic acidmdashwhich are the direct biosynthetic precursors of amino acids lysine and ornithine respectively These in turn produce cadaverine a ldquoC
5Nrdquo unit
and putrescine a ldquoC4Nrdquo unit which are major components
for the biosynthesis of important alkaloids as will be discussed later (Scheme 16) Additionally ornithine is a precursor of arginine another important amino acid
ornithine‐derived alkaloids (incorporating the c4n
unit) Putrescine is derived from the decarboxylation of ornithine and is a precursor of linear polyamines like spermshyine After enzymatic methylation of one amine of putrescine
C10 C15
C15
C10
C10
C30
CO2H
Chrysanthemic acid
OH
Menthol
O
HO
H
HOGlc
MeO2CLoganin (iridoid)
H
H
H
HH
OO
OParthenolide
O
O
OHOHO
OAcHO
OnC3H7
O
O
O
nC7H15
Thapsigargin
O
O
OO
H
H
O
Cleavage
H
H
Artemisinin
C20 C40
O
O
OO
OO
OHOHH
Bilobalide(highly rearrangedand missing 5 C)
CleavageH
Highoxidativecleavages
HO OAcH
AcO O OH
OOOBz
O
Ph
BzNH
OHFrom PHE Paclitaxel
C25O
H
HO
OHCH
OH Ophiobolin A
H
HO
H
H
HHCholesterol
(missing 3 C WM)
H
H H H
H
H
Hopene
β-Carotene
C30 - 3
C15
C20 - 5WM
WM
FIGURE 13 Chemical diversity in the terpene series (Wm Wagnerndashmeerwein shifts lost carbons bold bonds are remnant of primary building blocks)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 7
in the presence of S‐adenosylmethionine transamination of the other affords γ‐(N‐methylamino)aldehyde [20] The resulting cyclic iminium is a key intermediate in the formation of many medicinally important alkaloids such as
the plant‐derived compounds cocaine atropine or the calysshytegines [21 22] Indeed this iminium is a mannich acceptor which can react with various nucleophiles the first of those being the carbanion of acetyl‐CoA Thus after a stepwise
CO2H
RCinnamic acid (R=H)p-Coumaric acid (R=OH)
RO
PHETYR
OH
[H] [O]
lignans
OH
O
O
O
O
OMeMeO
OMe
C mdash C andor C mdash Oradical couplings
After E Zisomerization
for example Podophyllotoxin
O ORO
Coumarins(eg scopoletin)
[O]
Prenylationcyclizationcleavage[O]
O OO
Furocoumarins(eg psoralen)
RO
nAcetyl-CoA
thencyclization
n = 2 styrylpyrones
O O
OMe
n = 3 avonoids(from chalcone synthase)
C6C3
H
H H
From AcCoA
O
n = 3 stilbenoids(from stilbene synthase)
HO
OH
OHFrom AcCoA From AcCoA(ndashCO2)
Resveratrol
HO
OH
OH
OH
OH
H
Catechin
MeO Yangonin
From AcCoA
SCHEME 15 The phenylpropanoid biosynthetic pathways
LYS
CO2
Cadaverine
O NH
H2O
NH
Piperideineiminium
Pelletierine
AcAcCoA
CO2
NH
O
N
O
Pseudopelletierine
NH
Ph
OH
Ph
O
Lobeline
NH
δ+
δndash
N
H
H
N
H
OH
Lupinine
N
H N
H
(+)-Sparteine
N
NH
O (ndash)-Cytisine
NH
CO2H
Pipecolic acid
N
OHHO
OH
HO
H
Castanos permine
ORN
CO2
NH2 NH2 NH2 NH2
NH2
NH2
NH2
PutrescineO
NHMe
N-Methylpyrrolinium
2 timesAcCoA
NMe
ARG
n = 1 spermidinen = 2 homospermidine
NMe
O
HygrineCO2
CO2
MeN
O
[O]
TropinoneCO2
HNHO
OHOH
OH
Calystegine B2
MeN
OAtropine
O
Ph
HO
NMe
NMe
NMe
O
Cuscohygrine
HN
HN
NH2
( )n
O
n = 2
N
HHO
Retronecine
HH
H
H
H
HO
H H
H
SCHEME 16 Lysine‐ and ornithine‐derived alkaloid biosynthetic pathways (mind the structural similarities)
8 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
elongation by two AcCoA units either decarboxylation can occur leading to the acetonylpyrrolidine hygrine or a secshyond mannich reaction by the intramolecular attack of the acetoacetate anion onto an oxidation‐derived pyrrolinium leading to the tropane skeleton (tropinone) The acetoacetyl‐CoA intermediate can also react intermolecularly with another pyrrolinium cation leading to cuscohygrine after decarboxylation Finally the pyrrolizidine alkaloids [23] are derived from homospermidine which when submitted to terminal oxidative deamination leads to the bicyclic skelshyeton of retronecine and further Senecio alkaloids We can mention herein that ornithine is a biosynthetic precursor of arginine bearing a guanidine function which is an intermediate toward the toxic compounds tetrodotoxin and saxitoxin (not shown)
lysine‐derived alkaloids (incorporating the c5n
unit) From lysine to piperidine alkaloids the biosynthetic steps parallel the one previously described from ornithine Indeed the oxidative deamination of cadaverine affords a δ‐amino aldehyde which cyclizes through imine formation into piperideine Protonation results in a mannich acceptor which is able to react with various nucleophiles such as β‐ketothioester anions The first product of these reactions is pelletierine which can further react through an intramolecular mannich
reaction leading to pseudopelletierine Quinolizidines [24] can also be formed first from the mannich reaction of the piperideine acceptor with the corresponding enamine nucleoshyphile and then after additional transformation steps leading for example to lupinine sparteine or cytisine
Indolizidine alkaloids [15] such as castanospermine and swainsonine are formed from pipecolic acid an amino acid derived from lysine which can be elongated by malonyl‐CoA followed by ring closure When protonated these alkaloids are oxonium mimics strongly inhibiting glycosidases
Tyrosine‐ and Phenylalanine‐Derived Alkaloids Tyrosine and phenylalanine amino acids are bearing the phenylshyethylamine moiety of many medicinally relevant alkaloids Further hydroxylations on the aromatic carbocycle or on the aliphatic part can be observed methylations can occur on phenolic oxygens and on the amine leading to catecholamines (adrenaline noradrenaline dopamine) Arylethylamines are also usual to react with endogenous aldehydes through PictetndashSpengler reactions [25] leading to important biosynthetic intermediates (Scheme 17) like
bull Reticuline from the reaction with 4‐hydroxyphenylacetshyaldehyde toward benzyltetrahydroisoquinoline alkaloids
NH2
Phenylethylamines
RONH TetrahydroisoquinolinesRO
RʹCHO
Rʹ
NH
(CH2)n
MeO
HO
Benzyltetrahydroisoquinoline(n = 1) for example reticuline
orPhenethyltetrahydroisoquinoline
(n = 2) eg autumnaline
IntermolecularCmdashO phenol
couplings
Curarealkaloids
IntramolecularC mdash C phenol
couplings
ONMe
HHO
H
HO
Morphine
NMe
MeO
HO
MeOOH
Isoboldine
O
O
OMe
NO2
CO2H
Aristolochic acid
OMe
O
NHAcMeO
MeOMeO
Colchicine
N
O
O
OMe
OMeBerberine
IntramolecularCmdash C coupling
and furtheroxidative events
Rʹ derivedfrom PHE
Pictet-Spenglerreaction
TYR
HO
HO CHO
HNHO
HO
OH
OMeO
OH
NMe
[O]
GalantamineNorbelladine
Rʹ derived fromsecologanin
NAc
HO
HO
O
CO2MeH
H
H
OHIpecosideaglycone
Ipecac alkaloids
1ClostCmdash C cleavage
and ring extension
H
ROH
1C lost
Cleavage
SCHEME 17 Tyrosine‐derived alkaloid biosynthetic pathways (double head arrows figure bond cleavages during biosynthetic processes)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 9
morphine berberine tubocurarine isoboldine or the highly modified aristolochic acid [26 27]
bull Automnaline from the reaction with 3‐(4‐hydroxyphenyl)propanal toward phenylethyltetrahydroisoquinoline alkaloids colchicine cephalotaxine or schelhammerishycine [28 29]
bull Ipecoside from the reaction of dopamine with secoloshyganin toward terpene tetrahydroisoquinoline alkaloids ipecoside or emetine
Lastly norbelladine (top of Scheme 17) is issued from the reductive amination of 34‐dihydroxybenzaldehyde (derived from phenylalanine) with tyramine (derived from tyrosine) and constitutes a biosynthetic node leading to Amaryllidaceae alkaloids such as galantamine crinine or lycorine depending on the topology of phenolic couplings In all these biosynthetic routes radical phenolic couplings are key reactions for CC and CO bond formations and rearrangements [30 31]
Tryptophan‐Derived Indole and Indole Monoterpene Alkaloids As for alkaloids derived from tyrosine and phenylalanine those derived from tryptophan are formed after decarboxylation of the amino acid (into tryptamine) and possible hydroxylation of the aromatic carbocycle (eg serotonin) and N‐methylation (eg psilocin) As previously tryptamine can also react through PictetndashSpengler reactions to form tetrahydro‐β‐carbolines which can be aromatized for example into harmine (Scheme 18) [16]
When the aldehyde partner of the PictetndashSpengler reacshytion with tryptamine is the terpene secologanin strictosidine is formed as an entry toward the vast monoterpene indole alkaloids [32 33] Hydrolysis of the glucosidic part releases the strictosidine aglycone bearing an aldehyde while iminshyium formation and further cyclization and reduction can lead to ajmalicine (from oxocyclization) or yohimbine (from carshybocyclization) These alkaloids are referred to as from the Corynanthe type with the monoterpene carbon skeleton unmodified Although it misses one carbon and has a very
RO
Carbonylcompounds
TRPNH
NH2
Indolethylamines(eg serotonine)
RONH
NH
Rʹ NH
N
MeOPictet-Spenglerreaction for example Harmineβ-Carbolines
NH
NHH
O
MeO2C
MeO2C
OH
H
H
Rʹ derived from secologanin
Strictosidine aglycone
NH
N
OH
H
H
Ajmalicine
N
N
O
O
H
H
H
Strychnine (C lost)Corynanthe type
Aspidosperma type Iboga type
N
N
OH
OMe
Quinine (C lost)
N
NH CO2Me
Catharanthine
NH
N
CO2MeH
OAc
OH
Vindoline
Complextransformations
NH
NMe
HN
MeN
H
H
Chimonanthine
Cyclizationdimerization
Prenylationcyclization
NH
NMeHO2C
H
Lysergic acid
H
H
Ergot alkaloidsPyrroloindoles Indole monoterpene
1C lost
1C lostFrom AcCoA
SCHEME 18 Tryptophan‐derived alkaloid biosynthetic pathways (gray parts monoterpenic units)
10 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
different structure strychnine is related to the Corynanthe alkaloids incorporating two carbons from acetyl‐CoA Highly modified monoterpene skeletons are derived from the Corynanthe core through CC bond breaking and reorshyganization leading to Iboga‐type (eg catharanthine) and Aspidosperma‐type (eg vindoline) alkaloids The antishycancer drug vinblastine is a heterodimer resulting from the nucleophilic attack of vindoline on a mannich acceptor resulting from catharanthine found in madagascar perishywinkle (Catharanthus roseus) The heteroaromatic comshypounds ellipticine camptothecin and quinine are also derived from a Corynanthe‐type precursor although in this case the biosynthetic relationship may not be obvious due to deep modifications of the skeleton
Finally two important classes of compounds have to be mentioned since they have inspired many synthetic chemists The pyrroloindole alkaloids result from the cyclization of tryptamine as found in physostigmine (formed by a cationic mechanism after methylation in position 3 of the indole not shown) or in chimonanthine (presumably formed by a radical coupling mechanism Scheme 18) The ergot alkaloids are derived from the 33‐dimethylallylation on position 4 of the indole in trypshytophan whose further cyclization and oxidation processes afford the natural products (eg lysergic acid Scheme 18 and ergotamine) which have had important medical applications [34]
NRPS Metabolites and Peptides NRPS enzymes assemble amino acids including nonproteinogenic ones into oligopeptides The enzymes contain several modules and especially an adenylation domain (A) which specifically selects and activates the amino acid to be transferred as a thioester on the nearby peptidyl carrier protein (PCP) [2] A condensation module (C) then catalyzes the formation of the peptide bonds between the newly introduced amino acyl‐PCP (bearing a free amine) and the elongated peptidyl‐PCP thioester At the end of the elongation a cyclization can occur into cyclopeptides but the peptide can also be
transferred to auxiliary enzymes like methyltransferases glycosyltransferases or oxidases (vancomycins are typical products of such functionalizations) [35 36] The formation of heterocycles is also frequently encountered in this metabolism as in penicillins that are derived from the tripeptide α‐aminoadipoyl‐cysteinyl‐valine or telomestatin (Fig 14) [2]
Other Alkaloid Origins There are many other nitrogen sources involved in alkaloid biosyntheses for example nicotinic acid (originated from aspartic acid and intershymediate in nicotine and anabasine biosyntheses) and anthranilic acid (originated from tryptophan and intermedishyate toward acridines or aurachins) The amination reaction (eg through transamination of carbonyl compounds) is also a way to introduce nitrogens in natural products for example from fatty acids steroids (toward Solanum alkaloids or cyclopamines) or other terpenoids (aconitine and atisine have diterpene skeletons while Daphniphyllum alkaloids are triterpene derivatives) Finally nucleic acids can also be precursors of alkaloids like the well‐known caffeine
12 FROM BIOSYNTHESIS TO TOTAL SYNTHESIS STRATEGIES TOWARD THE NATURAL PRODUCT CHEMICAL SPACE
121 The Chemical Space of Natural Products
Natural products occupy an important place in human com munities as demonstrated by their vast use from ancient times to nowadays like dyes fibers oils pershyfumes agrochemicals or drugs Broadly both primary and secondary metabolites could be classified as natural products while the latter as discussed previously are usushyally regarded as the ldquonatural productsrdquo owing to their comshyplexity and diversity arising from a variety of biosynthetic pathways The structural chemical diversity found among
N
S
CO2HO
HNPhH2C
O
Penicillin G
OH
HN
HN
O
O
ONH2
O
OHO
NHMe
OCl
O
NH
NH
NH
O
ClHO
O
HN
H
H
HOOH
OHHO2C
Vancomycin aglycone
ON
NO
O
NN
O
O
N
N O
Me
S
N N
OMeH
Telomestatin
L-AAA-L-CYS-D-VAL
Enzymes
FIGURE 14 Structural diversity of nonribosomal peptide compounds (AAA α‐aminoadipic acid)
FROm BIOSyNTHESIS TO TOTAL SyNTHESIS STRATEGIES TOWARD THE NATuRAL PRODuCT CHEmICAL SPACE 11
all living organisms defining the chemical space of natural products [37] is the consequence of their evolution occurshyring as an adaptation of organisms to their environment It is commonly believed that secondary metabolites are proshyduced as messengers by living organisms or as weapons against enemies and thus they should have certain biological activities in a medicinal point of view [38] Indeed natural products are regarded as one of the main sources of medicines (Fig 15) From the traditional medicinal extracts to every single bioactive molecule the methods of extraction purification identification and biological investigation of natural products have been well established Their complex structures and interesting properties have attracted synthetic chemists to accomplish their total syntheses and that of medicinally relevant anashylogs sometimes in the industrial context [39] Thus tarshygeting the chemical space of natural products has never been more relevant than today Although the discovery of natural products demands time and labor‐consuming manipulations it is worth to notice that the knowledge on this chemical space is still continually growing while biological advances allow for discovering and undershystanding potential targets However increasing the chemical space of human‐made compounds based on natural products should benefit from transdisciplinary colshylaborations such as the use of coupled biosynthetic and chemical synthetic methods to design original ldquounnatural natural productsrdquo [40]
122 The Biosynthetic Pathways as an Inspiration for Synthetic Challenges
1221 Precursor‐Directed Biosyntheses and Mutashysynthetic Strategies to Increase the Chemical Space of Natural Products As the genetics and biochemistry of natural product biosynthesis are better understood novel biosynthetic techniques have been developed to study and generate new diversity in natural product analogs (Scheme 19) Precursor‐directed biosynthesis (PDB) is considered as the earliest example of combining chemical and biological methods for the generation of complex natural product analogs [41 42] This approach compared with the biosynthetic pathway of wild‐type metabolites (Scheme 19a) involves the feeding of analogs of the natural biosynthetic building blocks to the living organisms (Scheme 19b) usually bacteria or fungi which incorporate the modified precursors into the biosynthesized compound mutasynthesis also termed as mutational biosynthesis (mBS) involves the inactivation of a key step of the bioshysynthesis in a mutant microorganism (Scheme 19c) which can then be fed by various modified or advanced building blocks (mutasynthons Scheme 19d) [43] These mutasshyynthons could not be incorporated by the wild type due to specificities of the enzymatic machinery Build up on PDB mBS eliminates the natural biosynthetic intermediate thus generating a less complex mixture of metabolites and making the purification or yield of target products better
O NH
OHO
O
OO
O
O
OH
HOO
O O
OH
HOO
NH ONH2
O
ONH
NH
O
OCl Cl
O
O
OH
HN
HN
HN
HN
OO
HO
HN
OH
OHOH
O
O
HO
HO
OHO
O
OHH2N
O
OH3C
H
O
H
CH3
CH3
H
O O
NO
H
O
HO
O
NO
O
NH
HN
OHO
HONH
ON
H
HO
O
H2N
O OH
ONH
OH
ON OHO
OH
HO OS OH
OOOH
OHO
O
ON
OO
O
HO
O
O OH
OO
Micafunginantifungal
Rapamycinimmunosuppressive
Galantamineanti-Alzheimer
Taxolanticancer
Artemisininantimalarial
Vancomycinantibiotic
FIGURE 15 Some famous natural products currently used as drugs
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product
CoNtENtS xi
SECTION IV MIXED BIOSYNTHETIC PATHWAYSndash THE STORY OF ALKALOIDS 381
11 Ornithine and Lysine Alkaloids 383Sebastian Brauch Wouter S Veldmate and Floris P J T Rutjes
111 Biosynthesis of l‐ornithine and l‐Lysine alkaloids 3831111 Biosynthetic Formation of alkaloids
Derived from l‐ornithine 3831112 Biosynthetic Formation of alkaloids
Derived from l‐Lysine 388112 the asymmetric Mannich Reaction in organic Synthesis 392
1121 Chiral amines as Catalysts in asymmetric Mannich Reactions 3941122 Chiral Broslashnsted Bases as Catalysts in asymmetric
Mannich Reactions 3981123 Chiral Broslashnsted acids as Catalysts in asymmetric
Mannich Reactions 4041124 organometallic Catalysts in asymmetric Mannich Reactions 4081125 Biocatalytic asymmetric Mannich Reactions 413
113 Mannich and Related Reactions in the total Synthesis of l‐Lysine‐ and l‐ornithine‐Derived alkaloids 414
114 Conclusion 426References 427
12 Tyrosine Alkaloids 431Uwe Rinner and Mario Waser
121 Introduction 431122 Biosynthesis of tyrosine‐Derived alkaloids 431
1221 Phenylethylamines 4311222 Simple tetrahydroisoquinoline alkaloids 4331223 Modified Benzyltetrahydroisoquinoline alkaloids 4331224 Phenethylisoquinoline alkaloids 4361225 amaryllidaceae alkaloids 4381226 Biosynthetic overview of tyrosine‐Derived alkaloids 442
123 arylndasharyl Coupling Reactions 4421231 Copper‐Mediated arylndasharyl Bond Forming Reactions 4431232 Nickel‐Mediated arylndasharyl Bond Forming Reactions 4461233 Palladium‐Mediated arylndasharyl Bond Forming Reactions 4471234 transition Metal‐Catalyzed Couplings of Nonactivated
aryl Compounds 450124 Synthesis of tyrosine‐Derived alkaloids 456
1241 Synthesis of Modified Benzyltetrahydroisoquinoline alkaloids 4561242 Synthesis of Phenethylisoquinoline alkaloids 4601243 Synthesis of amaryllidaceae alkaloids 462
125 Conclusion 468References 469
13 Histidine and Histidine‐Like Alkaloids 473Ian S Young
131 Introduction 473132 Biosynthesis 473133 atom Economy and Protecting‐Group‐Free Chemistry 480
xii CoNtENtS
134 Challenging the Boundaries of Synthesis PIas 488135 Conclusion 497References 499
14 Anthranilic AcidndashTryptophan Alkaloids 502Zhen‐Yu Tang
141 Biosynthesis 502142 Divergent SynthesisndashCollective total Synthesis 508143 Collective total Synthesis of tryptophan‐Derived alkaloids 510
1431 Monoterpene Indole alkaloids 5101432 Bisindole alkaloids 512
References 517
15 Future Directions of Modern Organic Synthesis 519Jakob Pletz and Rolf Breinbauer
151 Introduction 519152 Enzymes in organic Synthesis Merging total
Synthesis with Biosynthesis 520153 Engineered Biosynthesis 526154 Diversity‐oriented Synthesis Biology‐oriented Synthesis
and Diverted total Synthesis 5331541 Diversity‐oriented Synthesis 5351542 Biology‐oriented Synthesis 5361543 Diverted total Synthesis 539
155 Conclusion 541References 545
INDEX 548
Elissavet E Anagnostaki Department of Chemistry Laboratory of Organic Chemistry Aristotle University of Thessaloniki Thessaloniki Greece and Research and Development Department Pharmathen SA Thessaloniki Greece
Stellios Arseniyadis School of Biological and Chemical Sciences Queen Mary University of London London United Kingdom
Louis Barriault Department of Chemistry University of Ottawa Ottawa Ontario Canada
Erica Benedetti Laboratoire de Chimie et Biochimie et Pharmacologiques et Toxicologiques CNRS-Universiteacute Paris Descartes Faculteacute des Sciences Fondamentales et Biomeacutedicales Paris France
Sebastian Brauch Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
Rolf Breinbauer Institute of Organic Chemistry Technische Universitaumlt Graz Graz Austria
Cyril Bressy Aix Marseille Universiteacute Centrale Marseille CNRS Marseille France
Martin Cordes Institute for Organic Chemistry and Center of Biomolecular Drug Research (BMWZ) Leibniz Universitaumlt Hannover Hannover Germany and Helmholtz Center for Infection Research (HZI) Hannover Germany
Paul E Floreancig Department of Chemistry Chevron Science Center University of Pittsburgh Pittsburgh PA USA
Tobias A M Gulder Department of Chemistry and Center for Integrated Protein Science Munich (CIPSM) Biosystems Chemistry Technische Universitaumlt Muumlnchen Munich Germany
Trond Vidar Hansen School of Pharmacy University of Oslo Oslo Norway
Kazuaki Ishihara Department of Biotechnology Graduate School of Engineering Nagoya University Nagoya Japan
Markus Kalesse Institute for Organic Chemistry and Center of Biomolecular Drug Research (BMWZ) Leibniz Universitaumlt Hannover Hannover Germany and Helmholtz Center for Infection Research (HZI) Hannover Germany
Xu‐Wen Li Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
Bastien Nay Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France
Yu Peng State Key Laboratory of Applied Organic Chemistry Lanzhou University Lanzhou China
Jakob Pletz Institute of Organic Chemistry Technische Universitaumlt Graz Graz Austria
Uwe Rinner Institute of Organic Chemistry Johannes Kepler University Linz Linz Austria and Department of Chemistry College of Science Sultan Qaboos University Muscat Oman
Floris P J T Rutjes Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
LIST oF CoNTRIBUToRS
xiv LIST OF CONTRIBUTORS
Franccediloise Schaefers Department of Chemistry and Center for Integrated Protein Science Munich (CIPSM) Biosystems Chemistry Technische Universitaumlt Muumlnchen Munich Germany
Michael Smietana Institut des Biomoleacutecules Max Mousseron CNRS Universiteacute de Montpellier ENSCM France
Zhen‐Yu Tang Department of Pharmaceutical Engineering College of Chemistry and Chemical Engineering Central South University Changsha China
Wouter S Veldmate Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
Anders Vik School of Pharmacy University of Oslo Oslo Norway
Mario Waser Institute of Organic Chemistry Johannes Kepler University Linz Linz Austria
Youwei Xie Max‐Planck‐Institut fuumlr Kohlenforschung Muumllheim Germany
Ian S Young Bristol‐Myers Squibb Company Chemical Development New Brunswick NJ USA
Alexandros L Zografos Department of Chemistry Laboratory of Organic Chemistry Aristotle University of Thessaloniki Thessaloniki Greece
There is pleasure in the pathless woodsthere is rapture in the lonely shore
there is society where none intrudesby the deep sea and music in its roar
I love not Man the less but Nature moreLord Byron
Preface
The first time I came across with the idea of editing a book that merges selected chapters of biosynthesis and total synthesis was when I was teaching postgraduate courses of natural product synthesis at Aristotle University of Thessaloniki This period I realized that the best way to teach youngsters synthesis was to start from the very origin of inspiration nature and its tools biosynthesis
Over the last decades biosynthesis is filling our gaps of understanding the complex mechanisms of nature and pro-vides useful sources of inspiration not only in the way natural products can be synthesized but also by directing synthetic chemists in developing atom‐economical efficient synthetic methods Several are the examples that mimic biosynthetic guidelines from modern iterative alkylations and aldol reactions to CH oxidations that compile nowadays the modern toolbox of organic synthesis
The handed book is constructed in the logic of presenting the parallel development of biosynthesis and organic meth-odology and how these can be applied in efficient syntheses of natural products The book is divided into four sections each representing the four major biosynthetic pathways of natural products namely acetate mevalonate shikimate biosynthetic pathways and the mixed biosynthetic pathways
of alkaloids These sections are divided into chapters that represent selected classes of natural products for example lipids sesquiterpenoids lignans etc Each of these chapters is further divided into three distinct subchapters (a) biosyn-thesis (b) methodological section and (c) application of the described methodology in the total synthesis of the described family of natural products By this way the readers can be focused in the direct comparison between biosyn-theses and the developed methodologies to construct the crucial for each class of natural product carbon bonds Although the book as it develops is focused on presenting the power of biosynthesis and how this power can be applied in providing inspiration for the efficient synthesis of natural products it was not the authors will to present only biomi-metic total syntheses but rather to exploit the modern synthetic methodologies and recognize their disabilities for further improvement
Of course this book will not have been realized without the excellent work of renowned scientists worldwide working either in the field of biosynthesis or total synthesis who collected the existing knowledge on biosynthesis analyzed the existing modern methodologies and presented a bouquet of selected total syntheses Throughout our
xvi PrEfACE
endeavor to complete this book I learned many things from their expertise but I also realized that only with tight collab-orations you can build long‐lasting friendships I would like to thank them all once again for their trust and effort to complete this book We all hope that the current work will contribute to a better understanding of the current status of
organic chemistry and to the discovery of novel strategies and tactics for the synthesis of natural products
Alexandros L ZografosSeptember 2015
Thessaloniki Greece
From Biosynthesis to Total Synthesis Strategies and Tactics for Natural Products First Edition Edited by Alexandros L Zografoscopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
11 FROM PRIMARY TO SECONDARY METABOLISM THE KEY BUILDING BLOCKS
111 Definitions
The primary and secondary metabolisms are traditionally distinguished by their distribution and utility in the living organism network Primary metabolites include carbohyshydrates lipids nucleic acids and proteins (or their amino acid constituents) and are shared by all living organisms on Earth They are transformed by common pathways which are studied by biochemistry (Fig 11) Secondary metabo-lites are structurally diverse compounds usually produced by a limited number of organisms which synthesize them for a special purpose like defense or signaling through specific biosynthetic pathways They are studied by natural product chemistry This distinction is not always so obvious and some compounds can be studied in the context of both primary and secondary metabolisms This is especially true nowadays with the use of genetic and biomolecular tools which tend to make natural product sciences more and more integrative However an important point to remember is that the primary metabolism furnishes key building blocks to the secondary metabolism It would be difficult to describe in detail the full biosynthetic pathshyways in this section We tried to organize the discussion as a vade mecum synthetically gathering information from extremely useful sources which will be cited at the end of this chapter
112 Energy Supply and Carbon Storing at the Early Stage of Metabolisms
The sunlight is essential to life except in some part of the deep oceans It provides energy for plant photosynthesis that splits molecules of water into protons and electrons and releases O
2 (Scheme 11) A proton gradient inside the plant
chloroplasts then drags a transmembrane ATP synthase comshyplex that produces adenosine triphosphate (ATP) while elecshytrons released from water are transferred to the coenzyme reducer nicotinamide adenine dinucleotide phosphate hydride (NADPH) A major function of chloroplasts is to fix CO
2 as a combination to ribulose‐15‐bisphosphate (RuBP)
performed by RuBP carboxylase (rubisco) forming an instable ldquoC
6rdquo β‐ketoacid This is cleaved into two molecules
of 3‐phosphoglycerate (3‐PGA) which is then reduced into 3‐phosphoglyceraldehyde (3‐PGAL a ldquoC
3rdquo triose phosshy
phate) during the Calvin cycle This is one of the major metabolites in the biosynthesis of carbohydrates like glucose and a biochemical mean for storing and retaining carbon atoms in the living cells
113 Glucose as a Starting Material Toward Key Building Blocks of the Secondary Metabolism
Glucose‐6‐phosphate arises from the phosphorylation of glucose It is the starting material of glycolysis an important process of the primary metabolism which consists in eight enzymatic reactions leading to pyruvic acid (PA)
FROM BIOSYNTHESES TO TOTAL SYNTHESES AN INTRODUCTION
Bastien Nay1 and Xu‐Wen Li2
1
1 Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France2 Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
2 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
(Scheme 12) Important intermediates for the secondary metabolism are produced during glycolysis Glucose glucose‐6‐phosphate and fructose‐6‐phosphate can be converted to other hexoses and pentoses that can be oligoshymerized and enter in the composition of heterosides Additionally fructose‐6‐phosphate connects the pentose
phosphate pathway leading to erythrose‐4‐phosphate toward shikimic acid which is a key metabolite in the biosynthesis of aromatic amino acids (phenylalanine tyrosine or C
6C
3
units) and C6C
1 phenolic compounds The next important
intermediate in glycolysis is 3‐PGAL which can be redishyrected toward methylerythritol‐4‐phosphate (mEP) in the
Primary metabolism Secondary metabolism
The field ofbiochemistry
The field ofnatural product
chemistry
Essential toliving organisms
Essential to theproducer organisms
under particularconditions
Nucleic acids (DNARNA) carbohydrates
lipids amino acidspeptides and proteins
Alkaloids terpenespolyketides
polyphenols andtheir heterosidic form
Biological effects(defense signaling)
Biosyntheticpathways
Main compoundclasses
FIGURE 11 Primary versus secondary metabolisms
PS-I
PS-IIendash
hν H2O H+ and O2
ATP synthase
NADP reductase
H+ (gradient)
H+ADP + Pi ATP
NADPHH+NADP+
H+
Thylakoidcompartment
Chloroplaststroma
Thylakoidmembrane
(a) Light dependent process
(b) Light independent process
CO2
RuBP
Rubisco
Unstable β-ketoacid3-PGA 13-diPGA
ATP
ADP3-PGAL
NADPHH+
NADP+
(Calvin cycle)
trioses tetroses pentoses hexoses (eg glucose) heptoses
Chloroplaststroma
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
CH2OPO32ndash CH2OPO3
2ndash CH2OPO32ndash
CH2OPO32ndash
OOH
HO
HOO
HO
HO2CCO2H
HO
CO2PO32ndash
HO
CHOHO
endash
Cytosol
SCHEME 11 The photosynthetic machinery (PS‐I and PS‐II photosystems I and II)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 3
chloroplast mEP is a starting block in the biosynthesis of terpenes through C
5 isoprene units (isopentenyl diphosphate
(IPP) and dimethylallyl diphosphate (DmAPP)) especially those in C
10 C
20 and C
40 terpenes 3‐PGA is a precursor of
serine and other amino acids while phosphoenolpyruvate (PEP) the precursor of PA is also an intermediate toward the previously mentioned shikimic acid Lastly PA is not only a precursor of the fundamental ldquoC
2rdquo acetyl coenzyme A
(AcCoA) unit but also an intermediate toward aliphatic amino acids and mEP
AcCoA is the building block of fatty acids polyketides and mevalonic acid (mVA) a cytosolic precursor of the C
5 isoprene units for the biosynthesis of terpenes in the
C15
and C30
series (mind it is different from the mEP pathway in product and in cell location) Finally AcCoA enters the citric acid or Krebs cycle which leads to several precursors of amino acids These are oxaloacetic acid precursor of aspartic acid through transamination (thus toward lysine as a nitrogenated C
5N linear unit and methishy
onine as a methyl supplier) and 2‐oxoglutaric acid preshycursor of glutamic acid (and subsequent derivatives such as ornithine as a nitrogenated C
4N linear unit) All these
amino acids are key precursors in the biosynthesis of many alkaloids
114 Reactions Involved in the Construction of Secondary Metabolites
most reactions occurring in the living cells are performed by specialized enzymes which have been classified in an intershynational nomenclature defined by an enzyme commission (EC) number There are six classes of enzymes depending on the biochemical reaction they catalyze EC‐1 oxidoreducshytases (catalyzing oxidoreduction reactions) EC‐2 transfershyases (catalyzing the transfer of functional groups) EC‐3 hydrolases (catalyzing hydrolysis) EC‐4 lyases (breaking bonds through another process than hydrolysis or oxidation leading to a new double bond or a new cycle) EC‐5 isomershyases (catalyzing the isomerization of a molecule) and EC‐6 ligases (forming a covalent bond between two molecules) many subclasses of these enzymes have been described depending on the type of atoms and functional groups involved in the reaction and if any on the cofactor used in this reaction For example several cofactors can be used by dehydrogenases like NAD(P)NAD(P)H FADFADH
2 or
FmNFmNH2 For a description of this classification the
reader can refer to specialized Internet websites like ExplorEnz [1] What is important to realize is that most enzymes are substrate specific and have been selected during
CHOHO
CO2H
O
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
HOOH
HO
HOH2CO
OHC
CH2OPO32ndash
OHHO
CO2H
HO
OH
OH
TYR
PHE
CO2H
NH2
TRP
CH2OPO32ndash
HO
HOH2COH
MEP
C10 C20 and C40 terpenes
CO2HHO
SER
GLY CYS
CO2HOPO3
2ndash
CH2OH
OHHO2C
MVA
C15 and C30 terpenes
Polyketides
Krebscycle
(citric acid)
VAL ALA ILE LEU
CO2H
O
HO2C
CO2HO
CO2H
ASP
LYS
GLU
Glucose-6-phosphate
Fructose-6-phosphate
Erythrose-4-phosphate
3-PGAL
OP2O63ndash OP2O6
3ndash
IPP DMAPP
3-PGA PEP PA
O SCoAAcCoA
3x
Cytosol
Cytosol
Chloroplasts
Chloroplasts
Glycolysis
C2
C5
Shikimic acid Oxaloaceticacid
2-Oxoglutaricacid
MET
THR
PRO
ARGORN
Alkaloids Alkaloids Alkaloids Alkaloids
Aminoacids
Peptidesproteins
Phenolics
C1
C5N C4N
ldquoCH3rdquo
(Indole)C2NC6C3C6C2N C6C1
Pentosephosphatepathway
SCHEME 12 The building block chart involving glycolysis and the Krebs cycle
4 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
evolution to perform specific transformations making natural products with often and yet unknown functions
Secondary metabolites arise from specific biosynthetic pathways which use the previously defined building blocks The bunch of organic reactions involved in these biosynshytheses allows the construction of natural product frameshyworks which are finally diversified through ldquodecorationrdquo steps (Scheme 13) It is not the purpose of this introductive chapter to describe in detail all biosynthetic pathways and the reader can refer to excellent books and articles which have been published elsewhere [2 3]
The reactions involved in the construction of natural product skeletons will be described later for representative classes of compounds The identification of the building block footprint in the natural product skeleton will be emphasized as much as possible sometimes referring to biogenetic speculations [4] After the framework construction the decoration steps will involve as diverse reactions as aliphatic CH oxidations (eg involving a cytochrome P
450 oxygenase) occasionally triggering a
rearrangement heteroatom alkylations (eg methylation by S‐adenosylmethionine) or allylation (by DmAPP) esterifications heteroatom or C‐glycosylations (leading to heterosides) radical couplings (especially for phenols) alcohol oxidations or ketone reductions amineketone transaminations alkene dihydroxylations or epoxidations oxidative halogenations BaeyerndashVilliger oxidations and further oxygenation steps At the end of the biosynthesis such transformations may totally hide the primary building block origin of natural products
115 Secondary Metabolisms
1151 Polyketides Polyketides (or polyacetates) are issued from the oligomerization of C
2 acetate units performed
by polyketide synthases (PKS) and leading to (C2)
n linear
intermediates [5 6] If the (C2)
n intermediates arise from
successive Claisen reactions performed by ketosynthase
domains (KS in nonreducing PKS) a highly reactive poly‐ β‐ketoacyl intermediate H(CH
2CO)
nOH is formed
leading to phenolic and aromatic products through further intramolecular Claisen condensations Furthermore highly reducing PKSs are made of specialized enzymatic subunits working in line or iteratively to functionalize each C
2 linker
bond as CH(OH)CH2 (by ketoreductases (KR)) then as
HCCH (by dehydratases (DH)) and as CH2CH
2 (by enoyl
reductases (ER)) leading to a high degree of functionalization of the final product (Fig 12) By these iterative sequences highly reduced polyketides which can be either linear macrocyclized or polycyclized depending on the reactivity of the biosynthetic intermediates can be formed [7] With the same logic fatty acids are also biosynthesized by fatty acid synthases
moreover the PKS enzyme can be hybridized with nonshyribosomal peptide synthetase (NRPS) domains (see also ldquoNRPS metabolites and peptidesrdquo in the ldquoAlkaloidsrdquo secshytion) leading to the acylation of an amino acid by the (C
2)
n
acyl intermediate As previously the functionalization of the acyl chain depends on the PKS enzyme and the PKSNRPS products are also extremely diversified (eg hirsutellone B Fig 12) [8]
1152 Terpenes Terpenes are derived from the oligoshymerization of the C
5 isoprene units DmAPP and IPP Both
precursors are prompt to generate either an allylic cation (the diphosphate is a good leaving group) or a tertiary carbocation respectively which makes the IPP easy to react with DmAPP (Scheme 14) This reaction happens in the active site of a terpene synthase which activates the deparshyture of the diphosphate group from DmAPP thanks to Lewis acid activation (a metal like mg2+ mn2+ or Co2+ is present in the enzyme active site [9]) This leads to geranyl (C
10
monoterpene precursor) or farnesyl (C15
sesquiterpene precursor) diphosphate depending on the location of the enzyme (chloroplast for the mEP pathway or cytosol for the mVA pathway) Geranylgeranyl (C
20 diterpene) and
Constructionreactions
Decoration reaction(functionalization)Building
blocksNatural product
frameworksNatural products
HH
OH OAcH
HO O OH
OHO
OBz
P450
P450
P450
P450
P450 P450 Oxidativeauxiliaryenzymes
Taxadiene
OP2O63ndash
OP2O63ndash
3times
(a)
(b)
10-Deacetylbaccatin III
DMAPP
IPPand electrophilic
cyclizations
SCHEME 13 (a) From building blocks to natural products and (b) the example of 10‐deacetylbaccatin III
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 5
farnesylfarnesyl (C30
triterpene precursor) diphosphates can also be obtained by further additions of IPP leading to longer linear intermediates
The cyclization of linear precursors is achieved by speshycialized cyclases which generate a poorly functionalized natural product framework [10 11] Auxiliary enzymes such as oxygenases then increase the complexity and the diversity
of compounds by further functionalization (Scheme 13b) [12] A high degree of oxidation can be observed in comshypounds like thapsigargin paclitaxel or bilobalide (Fig 13) The biosynthesis of this last compound for example involves a high oxygenation pattern two Wagnerndashmeerwein rearrangements and several oxidative cleavages leading to the loss of five carbons The resulting natural products can
KS
S-ACP
O O
KR
S-ACP
OH O
S-MAT
O
YesNo
R
R
R
YesNoDH
S-ACP
OR
YesNoER
S-ACP
OR
YesNoTE
OH
OR
New cycle
New cycleor
release
New cycleor
release
New cycleor
release
Release
R in
crem
ente
d by
two
carb
ons
HO
O
S-ACP
O
Claisen condensation (ndashCO2)
reduction (NADPH)
dehydration (ndashH2O)
+
reduction (NADPH)
hydrolysis (H2O)
O O NH
OH
OH
H H
H H
Hirsutellone B(mixed PKSNRPS
product)
OO
O
OHOH
HO OH
Norsolorinic acid
O
O
O
O CHO
OH
HH
H
H
HH
OHHemibrevetoxin B
OOHO
OHO
HOHO OH
Erythronolide A
OH
OH
HO
Panaxytriol
From ahexanoylstartingblock
H
H
Fromtyrosine
H
1C lost fromdecarboxylation
FIGURE 12 Chemical logic of polyketide construction leading to variable functionalization of the elongated acyl chain and examples of resulting chemical diversity ACP acyl carrier protein DH dehydratase ER enoyl reductase KR ketoreductase KS ketosynthase mAT malonyl acyl transferase TE thioesterase
DMAPPOP2O6
3ndashOP2O6
3ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
Geranyl diphosphate (GPP)2 times IPP
H H
Geranylgeranyl diphosphate (GGPP)
Examplesverticillane
casbanetaxane
phorbol
Exampleslabdane
pimaranekauraneabietane
aphidicolanegibberellane
IPP
SCHEME 14 Early assembly of C5 units in terpene biosynthesis leading to diterpenes (C
20)
6 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
thus be extremely modified with structures whose biogenetic origin is far from being obvious at first sight and cannot be determined without further experiments such as isotopic labeling
1153 Flavonoids Resveratrols Gallic Acids and Further Polyphenolics We have previously discussed the polyketide origin of some phenolic compounds based on the (C
2)
n motif Other polyphenols like gallic acids are directly
derived by the aromatization of shikimic acid (C6C
1 building
block Scheme 12) [13] The C6C
3 building blocks are availshy
able from the conversion of phenylalanine and tyrosine into cinnamic and p‐coumaric acids respectively and then by further hydroxylation steps (Scheme 15) These can dimerize into lignans (eg podophyllotoxin) [14 15] through radical processes or converted to low molecular weight compounds like eugenol coumarins or vanillin [16] The coenzyme A thioesters of these C
6C
3 acids can be used as initiator units
by specialized ketosynthases for an elongation by two acetyl units leading to aromatic polyketides like styrylpyrones or diarylheptanoids (eg curcumin) [17] Important comshypounds from this metabolism are flavonoids (C
6C
3C
6) [18]
and stilbenoids (C6C
2C
6) (a decarboxylation occurs during
the aryl cyclization) [19] which are synthesized by chalcone synthase and stilbene synthase respectively Flavonoids (eg catechin) and stilbenes (eg resveratrol) are present in large amounts in fruits and vegetables and may exert their radical scavenging properties in vivo
1154 Alkaloids Alkaloids are nitrogen‐containing compounds The nitrogen(s) can be involved in an amine function conferring basicity to the natural product (like ldquoalkalirdquo) or in less or nonbasic functions such as an amide a nitrile an isonitrile or an ammonium salt (quaternary amines) For amines protonation often occurs at physiological pH and may condition their biological activity In many cases the nitrogen is biogenetically derived from an amino acid We will thus discuss alkaloids according to their amino acid origin
Alkaloids Derived from the Krebs Cycle (Lysine and Ornithine Derived) As shown previously (Scheme 12) the Krebs cycle is a crucial metabolic process which leads to α‐ketoacids (oxaloacetic and 2‐oxoglutaric acids) Their enzymatic transamination affords the two amino acidsmdashaspartic acid and glutamic acidmdashwhich are the direct biosynthetic precursors of amino acids lysine and ornithine respectively These in turn produce cadaverine a ldquoC
5Nrdquo unit
and putrescine a ldquoC4Nrdquo unit which are major components
for the biosynthesis of important alkaloids as will be discussed later (Scheme 16) Additionally ornithine is a precursor of arginine another important amino acid
ornithine‐derived alkaloids (incorporating the c4n
unit) Putrescine is derived from the decarboxylation of ornithine and is a precursor of linear polyamines like spermshyine After enzymatic methylation of one amine of putrescine
C10 C15
C15
C10
C10
C30
CO2H
Chrysanthemic acid
OH
Menthol
O
HO
H
HOGlc
MeO2CLoganin (iridoid)
H
H
H
HH
OO
OParthenolide
O
O
OHOHO
OAcHO
OnC3H7
O
O
O
nC7H15
Thapsigargin
O
O
OO
H
H
O
Cleavage
H
H
Artemisinin
C20 C40
O
O
OO
OO
OHOHH
Bilobalide(highly rearrangedand missing 5 C)
CleavageH
Highoxidativecleavages
HO OAcH
AcO O OH
OOOBz
O
Ph
BzNH
OHFrom PHE Paclitaxel
C25O
H
HO
OHCH
OH Ophiobolin A
H
HO
H
H
HHCholesterol
(missing 3 C WM)
H
H H H
H
H
Hopene
β-Carotene
C30 - 3
C15
C20 - 5WM
WM
FIGURE 13 Chemical diversity in the terpene series (Wm Wagnerndashmeerwein shifts lost carbons bold bonds are remnant of primary building blocks)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 7
in the presence of S‐adenosylmethionine transamination of the other affords γ‐(N‐methylamino)aldehyde [20] The resulting cyclic iminium is a key intermediate in the formation of many medicinally important alkaloids such as
the plant‐derived compounds cocaine atropine or the calysshytegines [21 22] Indeed this iminium is a mannich acceptor which can react with various nucleophiles the first of those being the carbanion of acetyl‐CoA Thus after a stepwise
CO2H
RCinnamic acid (R=H)p-Coumaric acid (R=OH)
RO
PHETYR
OH
[H] [O]
lignans
OH
O
O
O
O
OMeMeO
OMe
C mdash C andor C mdash Oradical couplings
After E Zisomerization
for example Podophyllotoxin
O ORO
Coumarins(eg scopoletin)
[O]
Prenylationcyclizationcleavage[O]
O OO
Furocoumarins(eg psoralen)
RO
nAcetyl-CoA
thencyclization
n = 2 styrylpyrones
O O
OMe
n = 3 avonoids(from chalcone synthase)
C6C3
H
H H
From AcCoA
O
n = 3 stilbenoids(from stilbene synthase)
HO
OH
OHFrom AcCoA From AcCoA(ndashCO2)
Resveratrol
HO
OH
OH
OH
OH
H
Catechin
MeO Yangonin
From AcCoA
SCHEME 15 The phenylpropanoid biosynthetic pathways
LYS
CO2
Cadaverine
O NH
H2O
NH
Piperideineiminium
Pelletierine
AcAcCoA
CO2
NH
O
N
O
Pseudopelletierine
NH
Ph
OH
Ph
O
Lobeline
NH
δ+
δndash
N
H
H
N
H
OH
Lupinine
N
H N
H
(+)-Sparteine
N
NH
O (ndash)-Cytisine
NH
CO2H
Pipecolic acid
N
OHHO
OH
HO
H
Castanos permine
ORN
CO2
NH2 NH2 NH2 NH2
NH2
NH2
NH2
PutrescineO
NHMe
N-Methylpyrrolinium
2 timesAcCoA
NMe
ARG
n = 1 spermidinen = 2 homospermidine
NMe
O
HygrineCO2
CO2
MeN
O
[O]
TropinoneCO2
HNHO
OHOH
OH
Calystegine B2
MeN
OAtropine
O
Ph
HO
NMe
NMe
NMe
O
Cuscohygrine
HN
HN
NH2
( )n
O
n = 2
N
HHO
Retronecine
HH
H
H
H
HO
H H
H
SCHEME 16 Lysine‐ and ornithine‐derived alkaloid biosynthetic pathways (mind the structural similarities)
8 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
elongation by two AcCoA units either decarboxylation can occur leading to the acetonylpyrrolidine hygrine or a secshyond mannich reaction by the intramolecular attack of the acetoacetate anion onto an oxidation‐derived pyrrolinium leading to the tropane skeleton (tropinone) The acetoacetyl‐CoA intermediate can also react intermolecularly with another pyrrolinium cation leading to cuscohygrine after decarboxylation Finally the pyrrolizidine alkaloids [23] are derived from homospermidine which when submitted to terminal oxidative deamination leads to the bicyclic skelshyeton of retronecine and further Senecio alkaloids We can mention herein that ornithine is a biosynthetic precursor of arginine bearing a guanidine function which is an intermediate toward the toxic compounds tetrodotoxin and saxitoxin (not shown)
lysine‐derived alkaloids (incorporating the c5n
unit) From lysine to piperidine alkaloids the biosynthetic steps parallel the one previously described from ornithine Indeed the oxidative deamination of cadaverine affords a δ‐amino aldehyde which cyclizes through imine formation into piperideine Protonation results in a mannich acceptor which is able to react with various nucleophiles such as β‐ketothioester anions The first product of these reactions is pelletierine which can further react through an intramolecular mannich
reaction leading to pseudopelletierine Quinolizidines [24] can also be formed first from the mannich reaction of the piperideine acceptor with the corresponding enamine nucleoshyphile and then after additional transformation steps leading for example to lupinine sparteine or cytisine
Indolizidine alkaloids [15] such as castanospermine and swainsonine are formed from pipecolic acid an amino acid derived from lysine which can be elongated by malonyl‐CoA followed by ring closure When protonated these alkaloids are oxonium mimics strongly inhibiting glycosidases
Tyrosine‐ and Phenylalanine‐Derived Alkaloids Tyrosine and phenylalanine amino acids are bearing the phenylshyethylamine moiety of many medicinally relevant alkaloids Further hydroxylations on the aromatic carbocycle or on the aliphatic part can be observed methylations can occur on phenolic oxygens and on the amine leading to catecholamines (adrenaline noradrenaline dopamine) Arylethylamines are also usual to react with endogenous aldehydes through PictetndashSpengler reactions [25] leading to important biosynthetic intermediates (Scheme 17) like
bull Reticuline from the reaction with 4‐hydroxyphenylacetshyaldehyde toward benzyltetrahydroisoquinoline alkaloids
NH2
Phenylethylamines
RONH TetrahydroisoquinolinesRO
RʹCHO
Rʹ
NH
(CH2)n
MeO
HO
Benzyltetrahydroisoquinoline(n = 1) for example reticuline
orPhenethyltetrahydroisoquinoline
(n = 2) eg autumnaline
IntermolecularCmdashO phenol
couplings
Curarealkaloids
IntramolecularC mdash C phenol
couplings
ONMe
HHO
H
HO
Morphine
NMe
MeO
HO
MeOOH
Isoboldine
O
O
OMe
NO2
CO2H
Aristolochic acid
OMe
O
NHAcMeO
MeOMeO
Colchicine
N
O
O
OMe
OMeBerberine
IntramolecularCmdash C coupling
and furtheroxidative events
Rʹ derivedfrom PHE
Pictet-Spenglerreaction
TYR
HO
HO CHO
HNHO
HO
OH
OMeO
OH
NMe
[O]
GalantamineNorbelladine
Rʹ derived fromsecologanin
NAc
HO
HO
O
CO2MeH
H
H
OHIpecosideaglycone
Ipecac alkaloids
1ClostCmdash C cleavage
and ring extension
H
ROH
1C lost
Cleavage
SCHEME 17 Tyrosine‐derived alkaloid biosynthetic pathways (double head arrows figure bond cleavages during biosynthetic processes)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 9
morphine berberine tubocurarine isoboldine or the highly modified aristolochic acid [26 27]
bull Automnaline from the reaction with 3‐(4‐hydroxyphenyl)propanal toward phenylethyltetrahydroisoquinoline alkaloids colchicine cephalotaxine or schelhammerishycine [28 29]
bull Ipecoside from the reaction of dopamine with secoloshyganin toward terpene tetrahydroisoquinoline alkaloids ipecoside or emetine
Lastly norbelladine (top of Scheme 17) is issued from the reductive amination of 34‐dihydroxybenzaldehyde (derived from phenylalanine) with tyramine (derived from tyrosine) and constitutes a biosynthetic node leading to Amaryllidaceae alkaloids such as galantamine crinine or lycorine depending on the topology of phenolic couplings In all these biosynthetic routes radical phenolic couplings are key reactions for CC and CO bond formations and rearrangements [30 31]
Tryptophan‐Derived Indole and Indole Monoterpene Alkaloids As for alkaloids derived from tyrosine and phenylalanine those derived from tryptophan are formed after decarboxylation of the amino acid (into tryptamine) and possible hydroxylation of the aromatic carbocycle (eg serotonin) and N‐methylation (eg psilocin) As previously tryptamine can also react through PictetndashSpengler reactions to form tetrahydro‐β‐carbolines which can be aromatized for example into harmine (Scheme 18) [16]
When the aldehyde partner of the PictetndashSpengler reacshytion with tryptamine is the terpene secologanin strictosidine is formed as an entry toward the vast monoterpene indole alkaloids [32 33] Hydrolysis of the glucosidic part releases the strictosidine aglycone bearing an aldehyde while iminshyium formation and further cyclization and reduction can lead to ajmalicine (from oxocyclization) or yohimbine (from carshybocyclization) These alkaloids are referred to as from the Corynanthe type with the monoterpene carbon skeleton unmodified Although it misses one carbon and has a very
RO
Carbonylcompounds
TRPNH
NH2
Indolethylamines(eg serotonine)
RONH
NH
Rʹ NH
N
MeOPictet-Spenglerreaction for example Harmineβ-Carbolines
NH
NHH
O
MeO2C
MeO2C
OH
H
H
Rʹ derived from secologanin
Strictosidine aglycone
NH
N
OH
H
H
Ajmalicine
N
N
O
O
H
H
H
Strychnine (C lost)Corynanthe type
Aspidosperma type Iboga type
N
N
OH
OMe
Quinine (C lost)
N
NH CO2Me
Catharanthine
NH
N
CO2MeH
OAc
OH
Vindoline
Complextransformations
NH
NMe
HN
MeN
H
H
Chimonanthine
Cyclizationdimerization
Prenylationcyclization
NH
NMeHO2C
H
Lysergic acid
H
H
Ergot alkaloidsPyrroloindoles Indole monoterpene
1C lost
1C lostFrom AcCoA
SCHEME 18 Tryptophan‐derived alkaloid biosynthetic pathways (gray parts monoterpenic units)
10 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
different structure strychnine is related to the Corynanthe alkaloids incorporating two carbons from acetyl‐CoA Highly modified monoterpene skeletons are derived from the Corynanthe core through CC bond breaking and reorshyganization leading to Iboga‐type (eg catharanthine) and Aspidosperma‐type (eg vindoline) alkaloids The antishycancer drug vinblastine is a heterodimer resulting from the nucleophilic attack of vindoline on a mannich acceptor resulting from catharanthine found in madagascar perishywinkle (Catharanthus roseus) The heteroaromatic comshypounds ellipticine camptothecin and quinine are also derived from a Corynanthe‐type precursor although in this case the biosynthetic relationship may not be obvious due to deep modifications of the skeleton
Finally two important classes of compounds have to be mentioned since they have inspired many synthetic chemists The pyrroloindole alkaloids result from the cyclization of tryptamine as found in physostigmine (formed by a cationic mechanism after methylation in position 3 of the indole not shown) or in chimonanthine (presumably formed by a radical coupling mechanism Scheme 18) The ergot alkaloids are derived from the 33‐dimethylallylation on position 4 of the indole in trypshytophan whose further cyclization and oxidation processes afford the natural products (eg lysergic acid Scheme 18 and ergotamine) which have had important medical applications [34]
NRPS Metabolites and Peptides NRPS enzymes assemble amino acids including nonproteinogenic ones into oligopeptides The enzymes contain several modules and especially an adenylation domain (A) which specifically selects and activates the amino acid to be transferred as a thioester on the nearby peptidyl carrier protein (PCP) [2] A condensation module (C) then catalyzes the formation of the peptide bonds between the newly introduced amino acyl‐PCP (bearing a free amine) and the elongated peptidyl‐PCP thioester At the end of the elongation a cyclization can occur into cyclopeptides but the peptide can also be
transferred to auxiliary enzymes like methyltransferases glycosyltransferases or oxidases (vancomycins are typical products of such functionalizations) [35 36] The formation of heterocycles is also frequently encountered in this metabolism as in penicillins that are derived from the tripeptide α‐aminoadipoyl‐cysteinyl‐valine or telomestatin (Fig 14) [2]
Other Alkaloid Origins There are many other nitrogen sources involved in alkaloid biosyntheses for example nicotinic acid (originated from aspartic acid and intershymediate in nicotine and anabasine biosyntheses) and anthranilic acid (originated from tryptophan and intermedishyate toward acridines or aurachins) The amination reaction (eg through transamination of carbonyl compounds) is also a way to introduce nitrogens in natural products for example from fatty acids steroids (toward Solanum alkaloids or cyclopamines) or other terpenoids (aconitine and atisine have diterpene skeletons while Daphniphyllum alkaloids are triterpene derivatives) Finally nucleic acids can also be precursors of alkaloids like the well‐known caffeine
12 FROM BIOSYNTHESIS TO TOTAL SYNTHESIS STRATEGIES TOWARD THE NATURAL PRODUCT CHEMICAL SPACE
121 The Chemical Space of Natural Products
Natural products occupy an important place in human com munities as demonstrated by their vast use from ancient times to nowadays like dyes fibers oils pershyfumes agrochemicals or drugs Broadly both primary and secondary metabolites could be classified as natural products while the latter as discussed previously are usushyally regarded as the ldquonatural productsrdquo owing to their comshyplexity and diversity arising from a variety of biosynthetic pathways The structural chemical diversity found among
N
S
CO2HO
HNPhH2C
O
Penicillin G
OH
HN
HN
O
O
ONH2
O
OHO
NHMe
OCl
O
NH
NH
NH
O
ClHO
O
HN
H
H
HOOH
OHHO2C
Vancomycin aglycone
ON
NO
O
NN
O
O
N
N O
Me
S
N N
OMeH
Telomestatin
L-AAA-L-CYS-D-VAL
Enzymes
FIGURE 14 Structural diversity of nonribosomal peptide compounds (AAA α‐aminoadipic acid)
FROm BIOSyNTHESIS TO TOTAL SyNTHESIS STRATEGIES TOWARD THE NATuRAL PRODuCT CHEmICAL SPACE 11
all living organisms defining the chemical space of natural products [37] is the consequence of their evolution occurshyring as an adaptation of organisms to their environment It is commonly believed that secondary metabolites are proshyduced as messengers by living organisms or as weapons against enemies and thus they should have certain biological activities in a medicinal point of view [38] Indeed natural products are regarded as one of the main sources of medicines (Fig 15) From the traditional medicinal extracts to every single bioactive molecule the methods of extraction purification identification and biological investigation of natural products have been well established Their complex structures and interesting properties have attracted synthetic chemists to accomplish their total syntheses and that of medicinally relevant anashylogs sometimes in the industrial context [39] Thus tarshygeting the chemical space of natural products has never been more relevant than today Although the discovery of natural products demands time and labor‐consuming manipulations it is worth to notice that the knowledge on this chemical space is still continually growing while biological advances allow for discovering and undershystanding potential targets However increasing the chemical space of human‐made compounds based on natural products should benefit from transdisciplinary colshylaborations such as the use of coupled biosynthetic and chemical synthetic methods to design original ldquounnatural natural productsrdquo [40]
122 The Biosynthetic Pathways as an Inspiration for Synthetic Challenges
1221 Precursor‐Directed Biosyntheses and Mutashysynthetic Strategies to Increase the Chemical Space of Natural Products As the genetics and biochemistry of natural product biosynthesis are better understood novel biosynthetic techniques have been developed to study and generate new diversity in natural product analogs (Scheme 19) Precursor‐directed biosynthesis (PDB) is considered as the earliest example of combining chemical and biological methods for the generation of complex natural product analogs [41 42] This approach compared with the biosynthetic pathway of wild‐type metabolites (Scheme 19a) involves the feeding of analogs of the natural biosynthetic building blocks to the living organisms (Scheme 19b) usually bacteria or fungi which incorporate the modified precursors into the biosynthesized compound mutasynthesis also termed as mutational biosynthesis (mBS) involves the inactivation of a key step of the bioshysynthesis in a mutant microorganism (Scheme 19c) which can then be fed by various modified or advanced building blocks (mutasynthons Scheme 19d) [43] These mutasshyynthons could not be incorporated by the wild type due to specificities of the enzymatic machinery Build up on PDB mBS eliminates the natural biosynthetic intermediate thus generating a less complex mixture of metabolites and making the purification or yield of target products better
O NH
OHO
O
OO
O
O
OH
HOO
O O
OH
HOO
NH ONH2
O
ONH
NH
O
OCl Cl
O
O
OH
HN
HN
HN
HN
OO
HO
HN
OH
OHOH
O
O
HO
HO
OHO
O
OHH2N
O
OH3C
H
O
H
CH3
CH3
H
O O
NO
H
O
HO
O
NO
O
NH
HN
OHO
HONH
ON
H
HO
O
H2N
O OH
ONH
OH
ON OHO
OH
HO OS OH
OOOH
OHO
O
ON
OO
O
HO
O
O OH
OO
Micafunginantifungal
Rapamycinimmunosuppressive
Galantamineanti-Alzheimer
Taxolanticancer
Artemisininantimalarial
Vancomycinantibiotic
FIGURE 15 Some famous natural products currently used as drugs
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product
xii CoNtENtS
134 Challenging the Boundaries of Synthesis PIas 488135 Conclusion 497References 499
14 Anthranilic AcidndashTryptophan Alkaloids 502Zhen‐Yu Tang
141 Biosynthesis 502142 Divergent SynthesisndashCollective total Synthesis 508143 Collective total Synthesis of tryptophan‐Derived alkaloids 510
1431 Monoterpene Indole alkaloids 5101432 Bisindole alkaloids 512
References 517
15 Future Directions of Modern Organic Synthesis 519Jakob Pletz and Rolf Breinbauer
151 Introduction 519152 Enzymes in organic Synthesis Merging total
Synthesis with Biosynthesis 520153 Engineered Biosynthesis 526154 Diversity‐oriented Synthesis Biology‐oriented Synthesis
and Diverted total Synthesis 5331541 Diversity‐oriented Synthesis 5351542 Biology‐oriented Synthesis 5361543 Diverted total Synthesis 539
155 Conclusion 541References 545
INDEX 548
Elissavet E Anagnostaki Department of Chemistry Laboratory of Organic Chemistry Aristotle University of Thessaloniki Thessaloniki Greece and Research and Development Department Pharmathen SA Thessaloniki Greece
Stellios Arseniyadis School of Biological and Chemical Sciences Queen Mary University of London London United Kingdom
Louis Barriault Department of Chemistry University of Ottawa Ottawa Ontario Canada
Erica Benedetti Laboratoire de Chimie et Biochimie et Pharmacologiques et Toxicologiques CNRS-Universiteacute Paris Descartes Faculteacute des Sciences Fondamentales et Biomeacutedicales Paris France
Sebastian Brauch Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
Rolf Breinbauer Institute of Organic Chemistry Technische Universitaumlt Graz Graz Austria
Cyril Bressy Aix Marseille Universiteacute Centrale Marseille CNRS Marseille France
Martin Cordes Institute for Organic Chemistry and Center of Biomolecular Drug Research (BMWZ) Leibniz Universitaumlt Hannover Hannover Germany and Helmholtz Center for Infection Research (HZI) Hannover Germany
Paul E Floreancig Department of Chemistry Chevron Science Center University of Pittsburgh Pittsburgh PA USA
Tobias A M Gulder Department of Chemistry and Center for Integrated Protein Science Munich (CIPSM) Biosystems Chemistry Technische Universitaumlt Muumlnchen Munich Germany
Trond Vidar Hansen School of Pharmacy University of Oslo Oslo Norway
Kazuaki Ishihara Department of Biotechnology Graduate School of Engineering Nagoya University Nagoya Japan
Markus Kalesse Institute for Organic Chemistry and Center of Biomolecular Drug Research (BMWZ) Leibniz Universitaumlt Hannover Hannover Germany and Helmholtz Center for Infection Research (HZI) Hannover Germany
Xu‐Wen Li Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
Bastien Nay Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France
Yu Peng State Key Laboratory of Applied Organic Chemistry Lanzhou University Lanzhou China
Jakob Pletz Institute of Organic Chemistry Technische Universitaumlt Graz Graz Austria
Uwe Rinner Institute of Organic Chemistry Johannes Kepler University Linz Linz Austria and Department of Chemistry College of Science Sultan Qaboos University Muscat Oman
Floris P J T Rutjes Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
LIST oF CoNTRIBUToRS
xiv LIST OF CONTRIBUTORS
Franccediloise Schaefers Department of Chemistry and Center for Integrated Protein Science Munich (CIPSM) Biosystems Chemistry Technische Universitaumlt Muumlnchen Munich Germany
Michael Smietana Institut des Biomoleacutecules Max Mousseron CNRS Universiteacute de Montpellier ENSCM France
Zhen‐Yu Tang Department of Pharmaceutical Engineering College of Chemistry and Chemical Engineering Central South University Changsha China
Wouter S Veldmate Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
Anders Vik School of Pharmacy University of Oslo Oslo Norway
Mario Waser Institute of Organic Chemistry Johannes Kepler University Linz Linz Austria
Youwei Xie Max‐Planck‐Institut fuumlr Kohlenforschung Muumllheim Germany
Ian S Young Bristol‐Myers Squibb Company Chemical Development New Brunswick NJ USA
Alexandros L Zografos Department of Chemistry Laboratory of Organic Chemistry Aristotle University of Thessaloniki Thessaloniki Greece
There is pleasure in the pathless woodsthere is rapture in the lonely shore
there is society where none intrudesby the deep sea and music in its roar
I love not Man the less but Nature moreLord Byron
Preface
The first time I came across with the idea of editing a book that merges selected chapters of biosynthesis and total synthesis was when I was teaching postgraduate courses of natural product synthesis at Aristotle University of Thessaloniki This period I realized that the best way to teach youngsters synthesis was to start from the very origin of inspiration nature and its tools biosynthesis
Over the last decades biosynthesis is filling our gaps of understanding the complex mechanisms of nature and pro-vides useful sources of inspiration not only in the way natural products can be synthesized but also by directing synthetic chemists in developing atom‐economical efficient synthetic methods Several are the examples that mimic biosynthetic guidelines from modern iterative alkylations and aldol reactions to CH oxidations that compile nowadays the modern toolbox of organic synthesis
The handed book is constructed in the logic of presenting the parallel development of biosynthesis and organic meth-odology and how these can be applied in efficient syntheses of natural products The book is divided into four sections each representing the four major biosynthetic pathways of natural products namely acetate mevalonate shikimate biosynthetic pathways and the mixed biosynthetic pathways
of alkaloids These sections are divided into chapters that represent selected classes of natural products for example lipids sesquiterpenoids lignans etc Each of these chapters is further divided into three distinct subchapters (a) biosyn-thesis (b) methodological section and (c) application of the described methodology in the total synthesis of the described family of natural products By this way the readers can be focused in the direct comparison between biosyn-theses and the developed methodologies to construct the crucial for each class of natural product carbon bonds Although the book as it develops is focused on presenting the power of biosynthesis and how this power can be applied in providing inspiration for the efficient synthesis of natural products it was not the authors will to present only biomi-metic total syntheses but rather to exploit the modern synthetic methodologies and recognize their disabilities for further improvement
Of course this book will not have been realized without the excellent work of renowned scientists worldwide working either in the field of biosynthesis or total synthesis who collected the existing knowledge on biosynthesis analyzed the existing modern methodologies and presented a bouquet of selected total syntheses Throughout our
xvi PrEfACE
endeavor to complete this book I learned many things from their expertise but I also realized that only with tight collab-orations you can build long‐lasting friendships I would like to thank them all once again for their trust and effort to complete this book We all hope that the current work will contribute to a better understanding of the current status of
organic chemistry and to the discovery of novel strategies and tactics for the synthesis of natural products
Alexandros L ZografosSeptember 2015
Thessaloniki Greece
From Biosynthesis to Total Synthesis Strategies and Tactics for Natural Products First Edition Edited by Alexandros L Zografoscopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
11 FROM PRIMARY TO SECONDARY METABOLISM THE KEY BUILDING BLOCKS
111 Definitions
The primary and secondary metabolisms are traditionally distinguished by their distribution and utility in the living organism network Primary metabolites include carbohyshydrates lipids nucleic acids and proteins (or their amino acid constituents) and are shared by all living organisms on Earth They are transformed by common pathways which are studied by biochemistry (Fig 11) Secondary metabo-lites are structurally diverse compounds usually produced by a limited number of organisms which synthesize them for a special purpose like defense or signaling through specific biosynthetic pathways They are studied by natural product chemistry This distinction is not always so obvious and some compounds can be studied in the context of both primary and secondary metabolisms This is especially true nowadays with the use of genetic and biomolecular tools which tend to make natural product sciences more and more integrative However an important point to remember is that the primary metabolism furnishes key building blocks to the secondary metabolism It would be difficult to describe in detail the full biosynthetic pathshyways in this section We tried to organize the discussion as a vade mecum synthetically gathering information from extremely useful sources which will be cited at the end of this chapter
112 Energy Supply and Carbon Storing at the Early Stage of Metabolisms
The sunlight is essential to life except in some part of the deep oceans It provides energy for plant photosynthesis that splits molecules of water into protons and electrons and releases O
2 (Scheme 11) A proton gradient inside the plant
chloroplasts then drags a transmembrane ATP synthase comshyplex that produces adenosine triphosphate (ATP) while elecshytrons released from water are transferred to the coenzyme reducer nicotinamide adenine dinucleotide phosphate hydride (NADPH) A major function of chloroplasts is to fix CO
2 as a combination to ribulose‐15‐bisphosphate (RuBP)
performed by RuBP carboxylase (rubisco) forming an instable ldquoC
6rdquo β‐ketoacid This is cleaved into two molecules
of 3‐phosphoglycerate (3‐PGA) which is then reduced into 3‐phosphoglyceraldehyde (3‐PGAL a ldquoC
3rdquo triose phosshy
phate) during the Calvin cycle This is one of the major metabolites in the biosynthesis of carbohydrates like glucose and a biochemical mean for storing and retaining carbon atoms in the living cells
113 Glucose as a Starting Material Toward Key Building Blocks of the Secondary Metabolism
Glucose‐6‐phosphate arises from the phosphorylation of glucose It is the starting material of glycolysis an important process of the primary metabolism which consists in eight enzymatic reactions leading to pyruvic acid (PA)
FROM BIOSYNTHESES TO TOTAL SYNTHESES AN INTRODUCTION
Bastien Nay1 and Xu‐Wen Li2
1
1 Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France2 Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
2 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
(Scheme 12) Important intermediates for the secondary metabolism are produced during glycolysis Glucose glucose‐6‐phosphate and fructose‐6‐phosphate can be converted to other hexoses and pentoses that can be oligoshymerized and enter in the composition of heterosides Additionally fructose‐6‐phosphate connects the pentose
phosphate pathway leading to erythrose‐4‐phosphate toward shikimic acid which is a key metabolite in the biosynthesis of aromatic amino acids (phenylalanine tyrosine or C
6C
3
units) and C6C
1 phenolic compounds The next important
intermediate in glycolysis is 3‐PGAL which can be redishyrected toward methylerythritol‐4‐phosphate (mEP) in the
Primary metabolism Secondary metabolism
The field ofbiochemistry
The field ofnatural product
chemistry
Essential toliving organisms
Essential to theproducer organisms
under particularconditions
Nucleic acids (DNARNA) carbohydrates
lipids amino acidspeptides and proteins
Alkaloids terpenespolyketides
polyphenols andtheir heterosidic form
Biological effects(defense signaling)
Biosyntheticpathways
Main compoundclasses
FIGURE 11 Primary versus secondary metabolisms
PS-I
PS-IIendash
hν H2O H+ and O2
ATP synthase
NADP reductase
H+ (gradient)
H+ADP + Pi ATP
NADPHH+NADP+
H+
Thylakoidcompartment
Chloroplaststroma
Thylakoidmembrane
(a) Light dependent process
(b) Light independent process
CO2
RuBP
Rubisco
Unstable β-ketoacid3-PGA 13-diPGA
ATP
ADP3-PGAL
NADPHH+
NADP+
(Calvin cycle)
trioses tetroses pentoses hexoses (eg glucose) heptoses
Chloroplaststroma
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
CH2OPO32ndash CH2OPO3
2ndash CH2OPO32ndash
CH2OPO32ndash
OOH
HO
HOO
HO
HO2CCO2H
HO
CO2PO32ndash
HO
CHOHO
endash
Cytosol
SCHEME 11 The photosynthetic machinery (PS‐I and PS‐II photosystems I and II)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 3
chloroplast mEP is a starting block in the biosynthesis of terpenes through C
5 isoprene units (isopentenyl diphosphate
(IPP) and dimethylallyl diphosphate (DmAPP)) especially those in C
10 C
20 and C
40 terpenes 3‐PGA is a precursor of
serine and other amino acids while phosphoenolpyruvate (PEP) the precursor of PA is also an intermediate toward the previously mentioned shikimic acid Lastly PA is not only a precursor of the fundamental ldquoC
2rdquo acetyl coenzyme A
(AcCoA) unit but also an intermediate toward aliphatic amino acids and mEP
AcCoA is the building block of fatty acids polyketides and mevalonic acid (mVA) a cytosolic precursor of the C
5 isoprene units for the biosynthesis of terpenes in the
C15
and C30
series (mind it is different from the mEP pathway in product and in cell location) Finally AcCoA enters the citric acid or Krebs cycle which leads to several precursors of amino acids These are oxaloacetic acid precursor of aspartic acid through transamination (thus toward lysine as a nitrogenated C
5N linear unit and methishy
onine as a methyl supplier) and 2‐oxoglutaric acid preshycursor of glutamic acid (and subsequent derivatives such as ornithine as a nitrogenated C
4N linear unit) All these
amino acids are key precursors in the biosynthesis of many alkaloids
114 Reactions Involved in the Construction of Secondary Metabolites
most reactions occurring in the living cells are performed by specialized enzymes which have been classified in an intershynational nomenclature defined by an enzyme commission (EC) number There are six classes of enzymes depending on the biochemical reaction they catalyze EC‐1 oxidoreducshytases (catalyzing oxidoreduction reactions) EC‐2 transfershyases (catalyzing the transfer of functional groups) EC‐3 hydrolases (catalyzing hydrolysis) EC‐4 lyases (breaking bonds through another process than hydrolysis or oxidation leading to a new double bond or a new cycle) EC‐5 isomershyases (catalyzing the isomerization of a molecule) and EC‐6 ligases (forming a covalent bond between two molecules) many subclasses of these enzymes have been described depending on the type of atoms and functional groups involved in the reaction and if any on the cofactor used in this reaction For example several cofactors can be used by dehydrogenases like NAD(P)NAD(P)H FADFADH
2 or
FmNFmNH2 For a description of this classification the
reader can refer to specialized Internet websites like ExplorEnz [1] What is important to realize is that most enzymes are substrate specific and have been selected during
CHOHO
CO2H
O
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
HOOH
HO
HOH2CO
OHC
CH2OPO32ndash
OHHO
CO2H
HO
OH
OH
TYR
PHE
CO2H
NH2
TRP
CH2OPO32ndash
HO
HOH2COH
MEP
C10 C20 and C40 terpenes
CO2HHO
SER
GLY CYS
CO2HOPO3
2ndash
CH2OH
OHHO2C
MVA
C15 and C30 terpenes
Polyketides
Krebscycle
(citric acid)
VAL ALA ILE LEU
CO2H
O
HO2C
CO2HO
CO2H
ASP
LYS
GLU
Glucose-6-phosphate
Fructose-6-phosphate
Erythrose-4-phosphate
3-PGAL
OP2O63ndash OP2O6
3ndash
IPP DMAPP
3-PGA PEP PA
O SCoAAcCoA
3x
Cytosol
Cytosol
Chloroplasts
Chloroplasts
Glycolysis
C2
C5
Shikimic acid Oxaloaceticacid
2-Oxoglutaricacid
MET
THR
PRO
ARGORN
Alkaloids Alkaloids Alkaloids Alkaloids
Aminoacids
Peptidesproteins
Phenolics
C1
C5N C4N
ldquoCH3rdquo
(Indole)C2NC6C3C6C2N C6C1
Pentosephosphatepathway
SCHEME 12 The building block chart involving glycolysis and the Krebs cycle
4 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
evolution to perform specific transformations making natural products with often and yet unknown functions
Secondary metabolites arise from specific biosynthetic pathways which use the previously defined building blocks The bunch of organic reactions involved in these biosynshytheses allows the construction of natural product frameshyworks which are finally diversified through ldquodecorationrdquo steps (Scheme 13) It is not the purpose of this introductive chapter to describe in detail all biosynthetic pathways and the reader can refer to excellent books and articles which have been published elsewhere [2 3]
The reactions involved in the construction of natural product skeletons will be described later for representative classes of compounds The identification of the building block footprint in the natural product skeleton will be emphasized as much as possible sometimes referring to biogenetic speculations [4] After the framework construction the decoration steps will involve as diverse reactions as aliphatic CH oxidations (eg involving a cytochrome P
450 oxygenase) occasionally triggering a
rearrangement heteroatom alkylations (eg methylation by S‐adenosylmethionine) or allylation (by DmAPP) esterifications heteroatom or C‐glycosylations (leading to heterosides) radical couplings (especially for phenols) alcohol oxidations or ketone reductions amineketone transaminations alkene dihydroxylations or epoxidations oxidative halogenations BaeyerndashVilliger oxidations and further oxygenation steps At the end of the biosynthesis such transformations may totally hide the primary building block origin of natural products
115 Secondary Metabolisms
1151 Polyketides Polyketides (or polyacetates) are issued from the oligomerization of C
2 acetate units performed
by polyketide synthases (PKS) and leading to (C2)
n linear
intermediates [5 6] If the (C2)
n intermediates arise from
successive Claisen reactions performed by ketosynthase
domains (KS in nonreducing PKS) a highly reactive poly‐ β‐ketoacyl intermediate H(CH
2CO)
nOH is formed
leading to phenolic and aromatic products through further intramolecular Claisen condensations Furthermore highly reducing PKSs are made of specialized enzymatic subunits working in line or iteratively to functionalize each C
2 linker
bond as CH(OH)CH2 (by ketoreductases (KR)) then as
HCCH (by dehydratases (DH)) and as CH2CH
2 (by enoyl
reductases (ER)) leading to a high degree of functionalization of the final product (Fig 12) By these iterative sequences highly reduced polyketides which can be either linear macrocyclized or polycyclized depending on the reactivity of the biosynthetic intermediates can be formed [7] With the same logic fatty acids are also biosynthesized by fatty acid synthases
moreover the PKS enzyme can be hybridized with nonshyribosomal peptide synthetase (NRPS) domains (see also ldquoNRPS metabolites and peptidesrdquo in the ldquoAlkaloidsrdquo secshytion) leading to the acylation of an amino acid by the (C
2)
n
acyl intermediate As previously the functionalization of the acyl chain depends on the PKS enzyme and the PKSNRPS products are also extremely diversified (eg hirsutellone B Fig 12) [8]
1152 Terpenes Terpenes are derived from the oligoshymerization of the C
5 isoprene units DmAPP and IPP Both
precursors are prompt to generate either an allylic cation (the diphosphate is a good leaving group) or a tertiary carbocation respectively which makes the IPP easy to react with DmAPP (Scheme 14) This reaction happens in the active site of a terpene synthase which activates the deparshyture of the diphosphate group from DmAPP thanks to Lewis acid activation (a metal like mg2+ mn2+ or Co2+ is present in the enzyme active site [9]) This leads to geranyl (C
10
monoterpene precursor) or farnesyl (C15
sesquiterpene precursor) diphosphate depending on the location of the enzyme (chloroplast for the mEP pathway or cytosol for the mVA pathway) Geranylgeranyl (C
20 diterpene) and
Constructionreactions
Decoration reaction(functionalization)Building
blocksNatural product
frameworksNatural products
HH
OH OAcH
HO O OH
OHO
OBz
P450
P450
P450
P450
P450 P450 Oxidativeauxiliaryenzymes
Taxadiene
OP2O63ndash
OP2O63ndash
3times
(a)
(b)
10-Deacetylbaccatin III
DMAPP
IPPand electrophilic
cyclizations
SCHEME 13 (a) From building blocks to natural products and (b) the example of 10‐deacetylbaccatin III
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 5
farnesylfarnesyl (C30
triterpene precursor) diphosphates can also be obtained by further additions of IPP leading to longer linear intermediates
The cyclization of linear precursors is achieved by speshycialized cyclases which generate a poorly functionalized natural product framework [10 11] Auxiliary enzymes such as oxygenases then increase the complexity and the diversity
of compounds by further functionalization (Scheme 13b) [12] A high degree of oxidation can be observed in comshypounds like thapsigargin paclitaxel or bilobalide (Fig 13) The biosynthesis of this last compound for example involves a high oxygenation pattern two Wagnerndashmeerwein rearrangements and several oxidative cleavages leading to the loss of five carbons The resulting natural products can
KS
S-ACP
O O
KR
S-ACP
OH O
S-MAT
O
YesNo
R
R
R
YesNoDH
S-ACP
OR
YesNoER
S-ACP
OR
YesNoTE
OH
OR
New cycle
New cycleor
release
New cycleor
release
New cycleor
release
Release
R in
crem
ente
d by
two
carb
ons
HO
O
S-ACP
O
Claisen condensation (ndashCO2)
reduction (NADPH)
dehydration (ndashH2O)
+
reduction (NADPH)
hydrolysis (H2O)
O O NH
OH
OH
H H
H H
Hirsutellone B(mixed PKSNRPS
product)
OO
O
OHOH
HO OH
Norsolorinic acid
O
O
O
O CHO
OH
HH
H
H
HH
OHHemibrevetoxin B
OOHO
OHO
HOHO OH
Erythronolide A
OH
OH
HO
Panaxytriol
From ahexanoylstartingblock
H
H
Fromtyrosine
H
1C lost fromdecarboxylation
FIGURE 12 Chemical logic of polyketide construction leading to variable functionalization of the elongated acyl chain and examples of resulting chemical diversity ACP acyl carrier protein DH dehydratase ER enoyl reductase KR ketoreductase KS ketosynthase mAT malonyl acyl transferase TE thioesterase
DMAPPOP2O6
3ndashOP2O6
3ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
Geranyl diphosphate (GPP)2 times IPP
H H
Geranylgeranyl diphosphate (GGPP)
Examplesverticillane
casbanetaxane
phorbol
Exampleslabdane
pimaranekauraneabietane
aphidicolanegibberellane
IPP
SCHEME 14 Early assembly of C5 units in terpene biosynthesis leading to diterpenes (C
20)
6 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
thus be extremely modified with structures whose biogenetic origin is far from being obvious at first sight and cannot be determined without further experiments such as isotopic labeling
1153 Flavonoids Resveratrols Gallic Acids and Further Polyphenolics We have previously discussed the polyketide origin of some phenolic compounds based on the (C
2)
n motif Other polyphenols like gallic acids are directly
derived by the aromatization of shikimic acid (C6C
1 building
block Scheme 12) [13] The C6C
3 building blocks are availshy
able from the conversion of phenylalanine and tyrosine into cinnamic and p‐coumaric acids respectively and then by further hydroxylation steps (Scheme 15) These can dimerize into lignans (eg podophyllotoxin) [14 15] through radical processes or converted to low molecular weight compounds like eugenol coumarins or vanillin [16] The coenzyme A thioesters of these C
6C
3 acids can be used as initiator units
by specialized ketosynthases for an elongation by two acetyl units leading to aromatic polyketides like styrylpyrones or diarylheptanoids (eg curcumin) [17] Important comshypounds from this metabolism are flavonoids (C
6C
3C
6) [18]
and stilbenoids (C6C
2C
6) (a decarboxylation occurs during
the aryl cyclization) [19] which are synthesized by chalcone synthase and stilbene synthase respectively Flavonoids (eg catechin) and stilbenes (eg resveratrol) are present in large amounts in fruits and vegetables and may exert their radical scavenging properties in vivo
1154 Alkaloids Alkaloids are nitrogen‐containing compounds The nitrogen(s) can be involved in an amine function conferring basicity to the natural product (like ldquoalkalirdquo) or in less or nonbasic functions such as an amide a nitrile an isonitrile or an ammonium salt (quaternary amines) For amines protonation often occurs at physiological pH and may condition their biological activity In many cases the nitrogen is biogenetically derived from an amino acid We will thus discuss alkaloids according to their amino acid origin
Alkaloids Derived from the Krebs Cycle (Lysine and Ornithine Derived) As shown previously (Scheme 12) the Krebs cycle is a crucial metabolic process which leads to α‐ketoacids (oxaloacetic and 2‐oxoglutaric acids) Their enzymatic transamination affords the two amino acidsmdashaspartic acid and glutamic acidmdashwhich are the direct biosynthetic precursors of amino acids lysine and ornithine respectively These in turn produce cadaverine a ldquoC
5Nrdquo unit
and putrescine a ldquoC4Nrdquo unit which are major components
for the biosynthesis of important alkaloids as will be discussed later (Scheme 16) Additionally ornithine is a precursor of arginine another important amino acid
ornithine‐derived alkaloids (incorporating the c4n
unit) Putrescine is derived from the decarboxylation of ornithine and is a precursor of linear polyamines like spermshyine After enzymatic methylation of one amine of putrescine
C10 C15
C15
C10
C10
C30
CO2H
Chrysanthemic acid
OH
Menthol
O
HO
H
HOGlc
MeO2CLoganin (iridoid)
H
H
H
HH
OO
OParthenolide
O
O
OHOHO
OAcHO
OnC3H7
O
O
O
nC7H15
Thapsigargin
O
O
OO
H
H
O
Cleavage
H
H
Artemisinin
C20 C40
O
O
OO
OO
OHOHH
Bilobalide(highly rearrangedand missing 5 C)
CleavageH
Highoxidativecleavages
HO OAcH
AcO O OH
OOOBz
O
Ph
BzNH
OHFrom PHE Paclitaxel
C25O
H
HO
OHCH
OH Ophiobolin A
H
HO
H
H
HHCholesterol
(missing 3 C WM)
H
H H H
H
H
Hopene
β-Carotene
C30 - 3
C15
C20 - 5WM
WM
FIGURE 13 Chemical diversity in the terpene series (Wm Wagnerndashmeerwein shifts lost carbons bold bonds are remnant of primary building blocks)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 7
in the presence of S‐adenosylmethionine transamination of the other affords γ‐(N‐methylamino)aldehyde [20] The resulting cyclic iminium is a key intermediate in the formation of many medicinally important alkaloids such as
the plant‐derived compounds cocaine atropine or the calysshytegines [21 22] Indeed this iminium is a mannich acceptor which can react with various nucleophiles the first of those being the carbanion of acetyl‐CoA Thus after a stepwise
CO2H
RCinnamic acid (R=H)p-Coumaric acid (R=OH)
RO
PHETYR
OH
[H] [O]
lignans
OH
O
O
O
O
OMeMeO
OMe
C mdash C andor C mdash Oradical couplings
After E Zisomerization
for example Podophyllotoxin
O ORO
Coumarins(eg scopoletin)
[O]
Prenylationcyclizationcleavage[O]
O OO
Furocoumarins(eg psoralen)
RO
nAcetyl-CoA
thencyclization
n = 2 styrylpyrones
O O
OMe
n = 3 avonoids(from chalcone synthase)
C6C3
H
H H
From AcCoA
O
n = 3 stilbenoids(from stilbene synthase)
HO
OH
OHFrom AcCoA From AcCoA(ndashCO2)
Resveratrol
HO
OH
OH
OH
OH
H
Catechin
MeO Yangonin
From AcCoA
SCHEME 15 The phenylpropanoid biosynthetic pathways
LYS
CO2
Cadaverine
O NH
H2O
NH
Piperideineiminium
Pelletierine
AcAcCoA
CO2
NH
O
N
O
Pseudopelletierine
NH
Ph
OH
Ph
O
Lobeline
NH
δ+
δndash
N
H
H
N
H
OH
Lupinine
N
H N
H
(+)-Sparteine
N
NH
O (ndash)-Cytisine
NH
CO2H
Pipecolic acid
N
OHHO
OH
HO
H
Castanos permine
ORN
CO2
NH2 NH2 NH2 NH2
NH2
NH2
NH2
PutrescineO
NHMe
N-Methylpyrrolinium
2 timesAcCoA
NMe
ARG
n = 1 spermidinen = 2 homospermidine
NMe
O
HygrineCO2
CO2
MeN
O
[O]
TropinoneCO2
HNHO
OHOH
OH
Calystegine B2
MeN
OAtropine
O
Ph
HO
NMe
NMe
NMe
O
Cuscohygrine
HN
HN
NH2
( )n
O
n = 2
N
HHO
Retronecine
HH
H
H
H
HO
H H
H
SCHEME 16 Lysine‐ and ornithine‐derived alkaloid biosynthetic pathways (mind the structural similarities)
8 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
elongation by two AcCoA units either decarboxylation can occur leading to the acetonylpyrrolidine hygrine or a secshyond mannich reaction by the intramolecular attack of the acetoacetate anion onto an oxidation‐derived pyrrolinium leading to the tropane skeleton (tropinone) The acetoacetyl‐CoA intermediate can also react intermolecularly with another pyrrolinium cation leading to cuscohygrine after decarboxylation Finally the pyrrolizidine alkaloids [23] are derived from homospermidine which when submitted to terminal oxidative deamination leads to the bicyclic skelshyeton of retronecine and further Senecio alkaloids We can mention herein that ornithine is a biosynthetic precursor of arginine bearing a guanidine function which is an intermediate toward the toxic compounds tetrodotoxin and saxitoxin (not shown)
lysine‐derived alkaloids (incorporating the c5n
unit) From lysine to piperidine alkaloids the biosynthetic steps parallel the one previously described from ornithine Indeed the oxidative deamination of cadaverine affords a δ‐amino aldehyde which cyclizes through imine formation into piperideine Protonation results in a mannich acceptor which is able to react with various nucleophiles such as β‐ketothioester anions The first product of these reactions is pelletierine which can further react through an intramolecular mannich
reaction leading to pseudopelletierine Quinolizidines [24] can also be formed first from the mannich reaction of the piperideine acceptor with the corresponding enamine nucleoshyphile and then after additional transformation steps leading for example to lupinine sparteine or cytisine
Indolizidine alkaloids [15] such as castanospermine and swainsonine are formed from pipecolic acid an amino acid derived from lysine which can be elongated by malonyl‐CoA followed by ring closure When protonated these alkaloids are oxonium mimics strongly inhibiting glycosidases
Tyrosine‐ and Phenylalanine‐Derived Alkaloids Tyrosine and phenylalanine amino acids are bearing the phenylshyethylamine moiety of many medicinally relevant alkaloids Further hydroxylations on the aromatic carbocycle or on the aliphatic part can be observed methylations can occur on phenolic oxygens and on the amine leading to catecholamines (adrenaline noradrenaline dopamine) Arylethylamines are also usual to react with endogenous aldehydes through PictetndashSpengler reactions [25] leading to important biosynthetic intermediates (Scheme 17) like
bull Reticuline from the reaction with 4‐hydroxyphenylacetshyaldehyde toward benzyltetrahydroisoquinoline alkaloids
NH2
Phenylethylamines
RONH TetrahydroisoquinolinesRO
RʹCHO
Rʹ
NH
(CH2)n
MeO
HO
Benzyltetrahydroisoquinoline(n = 1) for example reticuline
orPhenethyltetrahydroisoquinoline
(n = 2) eg autumnaline
IntermolecularCmdashO phenol
couplings
Curarealkaloids
IntramolecularC mdash C phenol
couplings
ONMe
HHO
H
HO
Morphine
NMe
MeO
HO
MeOOH
Isoboldine
O
O
OMe
NO2
CO2H
Aristolochic acid
OMe
O
NHAcMeO
MeOMeO
Colchicine
N
O
O
OMe
OMeBerberine
IntramolecularCmdash C coupling
and furtheroxidative events
Rʹ derivedfrom PHE
Pictet-Spenglerreaction
TYR
HO
HO CHO
HNHO
HO
OH
OMeO
OH
NMe
[O]
GalantamineNorbelladine
Rʹ derived fromsecologanin
NAc
HO
HO
O
CO2MeH
H
H
OHIpecosideaglycone
Ipecac alkaloids
1ClostCmdash C cleavage
and ring extension
H
ROH
1C lost
Cleavage
SCHEME 17 Tyrosine‐derived alkaloid biosynthetic pathways (double head arrows figure bond cleavages during biosynthetic processes)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 9
morphine berberine tubocurarine isoboldine or the highly modified aristolochic acid [26 27]
bull Automnaline from the reaction with 3‐(4‐hydroxyphenyl)propanal toward phenylethyltetrahydroisoquinoline alkaloids colchicine cephalotaxine or schelhammerishycine [28 29]
bull Ipecoside from the reaction of dopamine with secoloshyganin toward terpene tetrahydroisoquinoline alkaloids ipecoside or emetine
Lastly norbelladine (top of Scheme 17) is issued from the reductive amination of 34‐dihydroxybenzaldehyde (derived from phenylalanine) with tyramine (derived from tyrosine) and constitutes a biosynthetic node leading to Amaryllidaceae alkaloids such as galantamine crinine or lycorine depending on the topology of phenolic couplings In all these biosynthetic routes radical phenolic couplings are key reactions for CC and CO bond formations and rearrangements [30 31]
Tryptophan‐Derived Indole and Indole Monoterpene Alkaloids As for alkaloids derived from tyrosine and phenylalanine those derived from tryptophan are formed after decarboxylation of the amino acid (into tryptamine) and possible hydroxylation of the aromatic carbocycle (eg serotonin) and N‐methylation (eg psilocin) As previously tryptamine can also react through PictetndashSpengler reactions to form tetrahydro‐β‐carbolines which can be aromatized for example into harmine (Scheme 18) [16]
When the aldehyde partner of the PictetndashSpengler reacshytion with tryptamine is the terpene secologanin strictosidine is formed as an entry toward the vast monoterpene indole alkaloids [32 33] Hydrolysis of the glucosidic part releases the strictosidine aglycone bearing an aldehyde while iminshyium formation and further cyclization and reduction can lead to ajmalicine (from oxocyclization) or yohimbine (from carshybocyclization) These alkaloids are referred to as from the Corynanthe type with the monoterpene carbon skeleton unmodified Although it misses one carbon and has a very
RO
Carbonylcompounds
TRPNH
NH2
Indolethylamines(eg serotonine)
RONH
NH
Rʹ NH
N
MeOPictet-Spenglerreaction for example Harmineβ-Carbolines
NH
NHH
O
MeO2C
MeO2C
OH
H
H
Rʹ derived from secologanin
Strictosidine aglycone
NH
N
OH
H
H
Ajmalicine
N
N
O
O
H
H
H
Strychnine (C lost)Corynanthe type
Aspidosperma type Iboga type
N
N
OH
OMe
Quinine (C lost)
N
NH CO2Me
Catharanthine
NH
N
CO2MeH
OAc
OH
Vindoline
Complextransformations
NH
NMe
HN
MeN
H
H
Chimonanthine
Cyclizationdimerization
Prenylationcyclization
NH
NMeHO2C
H
Lysergic acid
H
H
Ergot alkaloidsPyrroloindoles Indole monoterpene
1C lost
1C lostFrom AcCoA
SCHEME 18 Tryptophan‐derived alkaloid biosynthetic pathways (gray parts monoterpenic units)
10 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
different structure strychnine is related to the Corynanthe alkaloids incorporating two carbons from acetyl‐CoA Highly modified monoterpene skeletons are derived from the Corynanthe core through CC bond breaking and reorshyganization leading to Iboga‐type (eg catharanthine) and Aspidosperma‐type (eg vindoline) alkaloids The antishycancer drug vinblastine is a heterodimer resulting from the nucleophilic attack of vindoline on a mannich acceptor resulting from catharanthine found in madagascar perishywinkle (Catharanthus roseus) The heteroaromatic comshypounds ellipticine camptothecin and quinine are also derived from a Corynanthe‐type precursor although in this case the biosynthetic relationship may not be obvious due to deep modifications of the skeleton
Finally two important classes of compounds have to be mentioned since they have inspired many synthetic chemists The pyrroloindole alkaloids result from the cyclization of tryptamine as found in physostigmine (formed by a cationic mechanism after methylation in position 3 of the indole not shown) or in chimonanthine (presumably formed by a radical coupling mechanism Scheme 18) The ergot alkaloids are derived from the 33‐dimethylallylation on position 4 of the indole in trypshytophan whose further cyclization and oxidation processes afford the natural products (eg lysergic acid Scheme 18 and ergotamine) which have had important medical applications [34]
NRPS Metabolites and Peptides NRPS enzymes assemble amino acids including nonproteinogenic ones into oligopeptides The enzymes contain several modules and especially an adenylation domain (A) which specifically selects and activates the amino acid to be transferred as a thioester on the nearby peptidyl carrier protein (PCP) [2] A condensation module (C) then catalyzes the formation of the peptide bonds between the newly introduced amino acyl‐PCP (bearing a free amine) and the elongated peptidyl‐PCP thioester At the end of the elongation a cyclization can occur into cyclopeptides but the peptide can also be
transferred to auxiliary enzymes like methyltransferases glycosyltransferases or oxidases (vancomycins are typical products of such functionalizations) [35 36] The formation of heterocycles is also frequently encountered in this metabolism as in penicillins that are derived from the tripeptide α‐aminoadipoyl‐cysteinyl‐valine or telomestatin (Fig 14) [2]
Other Alkaloid Origins There are many other nitrogen sources involved in alkaloid biosyntheses for example nicotinic acid (originated from aspartic acid and intershymediate in nicotine and anabasine biosyntheses) and anthranilic acid (originated from tryptophan and intermedishyate toward acridines or aurachins) The amination reaction (eg through transamination of carbonyl compounds) is also a way to introduce nitrogens in natural products for example from fatty acids steroids (toward Solanum alkaloids or cyclopamines) or other terpenoids (aconitine and atisine have diterpene skeletons while Daphniphyllum alkaloids are triterpene derivatives) Finally nucleic acids can also be precursors of alkaloids like the well‐known caffeine
12 FROM BIOSYNTHESIS TO TOTAL SYNTHESIS STRATEGIES TOWARD THE NATURAL PRODUCT CHEMICAL SPACE
121 The Chemical Space of Natural Products
Natural products occupy an important place in human com munities as demonstrated by their vast use from ancient times to nowadays like dyes fibers oils pershyfumes agrochemicals or drugs Broadly both primary and secondary metabolites could be classified as natural products while the latter as discussed previously are usushyally regarded as the ldquonatural productsrdquo owing to their comshyplexity and diversity arising from a variety of biosynthetic pathways The structural chemical diversity found among
N
S
CO2HO
HNPhH2C
O
Penicillin G
OH
HN
HN
O
O
ONH2
O
OHO
NHMe
OCl
O
NH
NH
NH
O
ClHO
O
HN
H
H
HOOH
OHHO2C
Vancomycin aglycone
ON
NO
O
NN
O
O
N
N O
Me
S
N N
OMeH
Telomestatin
L-AAA-L-CYS-D-VAL
Enzymes
FIGURE 14 Structural diversity of nonribosomal peptide compounds (AAA α‐aminoadipic acid)
FROm BIOSyNTHESIS TO TOTAL SyNTHESIS STRATEGIES TOWARD THE NATuRAL PRODuCT CHEmICAL SPACE 11
all living organisms defining the chemical space of natural products [37] is the consequence of their evolution occurshyring as an adaptation of organisms to their environment It is commonly believed that secondary metabolites are proshyduced as messengers by living organisms or as weapons against enemies and thus they should have certain biological activities in a medicinal point of view [38] Indeed natural products are regarded as one of the main sources of medicines (Fig 15) From the traditional medicinal extracts to every single bioactive molecule the methods of extraction purification identification and biological investigation of natural products have been well established Their complex structures and interesting properties have attracted synthetic chemists to accomplish their total syntheses and that of medicinally relevant anashylogs sometimes in the industrial context [39] Thus tarshygeting the chemical space of natural products has never been more relevant than today Although the discovery of natural products demands time and labor‐consuming manipulations it is worth to notice that the knowledge on this chemical space is still continually growing while biological advances allow for discovering and undershystanding potential targets However increasing the chemical space of human‐made compounds based on natural products should benefit from transdisciplinary colshylaborations such as the use of coupled biosynthetic and chemical synthetic methods to design original ldquounnatural natural productsrdquo [40]
122 The Biosynthetic Pathways as an Inspiration for Synthetic Challenges
1221 Precursor‐Directed Biosyntheses and Mutashysynthetic Strategies to Increase the Chemical Space of Natural Products As the genetics and biochemistry of natural product biosynthesis are better understood novel biosynthetic techniques have been developed to study and generate new diversity in natural product analogs (Scheme 19) Precursor‐directed biosynthesis (PDB) is considered as the earliest example of combining chemical and biological methods for the generation of complex natural product analogs [41 42] This approach compared with the biosynthetic pathway of wild‐type metabolites (Scheme 19a) involves the feeding of analogs of the natural biosynthetic building blocks to the living organisms (Scheme 19b) usually bacteria or fungi which incorporate the modified precursors into the biosynthesized compound mutasynthesis also termed as mutational biosynthesis (mBS) involves the inactivation of a key step of the bioshysynthesis in a mutant microorganism (Scheme 19c) which can then be fed by various modified or advanced building blocks (mutasynthons Scheme 19d) [43] These mutasshyynthons could not be incorporated by the wild type due to specificities of the enzymatic machinery Build up on PDB mBS eliminates the natural biosynthetic intermediate thus generating a less complex mixture of metabolites and making the purification or yield of target products better
O NH
OHO
O
OO
O
O
OH
HOO
O O
OH
HOO
NH ONH2
O
ONH
NH
O
OCl Cl
O
O
OH
HN
HN
HN
HN
OO
HO
HN
OH
OHOH
O
O
HO
HO
OHO
O
OHH2N
O
OH3C
H
O
H
CH3
CH3
H
O O
NO
H
O
HO
O
NO
O
NH
HN
OHO
HONH
ON
H
HO
O
H2N
O OH
ONH
OH
ON OHO
OH
HO OS OH
OOOH
OHO
O
ON
OO
O
HO
O
O OH
OO
Micafunginantifungal
Rapamycinimmunosuppressive
Galantamineanti-Alzheimer
Taxolanticancer
Artemisininantimalarial
Vancomycinantibiotic
FIGURE 15 Some famous natural products currently used as drugs
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product
Elissavet E Anagnostaki Department of Chemistry Laboratory of Organic Chemistry Aristotle University of Thessaloniki Thessaloniki Greece and Research and Development Department Pharmathen SA Thessaloniki Greece
Stellios Arseniyadis School of Biological and Chemical Sciences Queen Mary University of London London United Kingdom
Louis Barriault Department of Chemistry University of Ottawa Ottawa Ontario Canada
Erica Benedetti Laboratoire de Chimie et Biochimie et Pharmacologiques et Toxicologiques CNRS-Universiteacute Paris Descartes Faculteacute des Sciences Fondamentales et Biomeacutedicales Paris France
Sebastian Brauch Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
Rolf Breinbauer Institute of Organic Chemistry Technische Universitaumlt Graz Graz Austria
Cyril Bressy Aix Marseille Universiteacute Centrale Marseille CNRS Marseille France
Martin Cordes Institute for Organic Chemistry and Center of Biomolecular Drug Research (BMWZ) Leibniz Universitaumlt Hannover Hannover Germany and Helmholtz Center for Infection Research (HZI) Hannover Germany
Paul E Floreancig Department of Chemistry Chevron Science Center University of Pittsburgh Pittsburgh PA USA
Tobias A M Gulder Department of Chemistry and Center for Integrated Protein Science Munich (CIPSM) Biosystems Chemistry Technische Universitaumlt Muumlnchen Munich Germany
Trond Vidar Hansen School of Pharmacy University of Oslo Oslo Norway
Kazuaki Ishihara Department of Biotechnology Graduate School of Engineering Nagoya University Nagoya Japan
Markus Kalesse Institute for Organic Chemistry and Center of Biomolecular Drug Research (BMWZ) Leibniz Universitaumlt Hannover Hannover Germany and Helmholtz Center for Infection Research (HZI) Hannover Germany
Xu‐Wen Li Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
Bastien Nay Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France
Yu Peng State Key Laboratory of Applied Organic Chemistry Lanzhou University Lanzhou China
Jakob Pletz Institute of Organic Chemistry Technische Universitaumlt Graz Graz Austria
Uwe Rinner Institute of Organic Chemistry Johannes Kepler University Linz Linz Austria and Department of Chemistry College of Science Sultan Qaboos University Muscat Oman
Floris P J T Rutjes Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
LIST oF CoNTRIBUToRS
xiv LIST OF CONTRIBUTORS
Franccediloise Schaefers Department of Chemistry and Center for Integrated Protein Science Munich (CIPSM) Biosystems Chemistry Technische Universitaumlt Muumlnchen Munich Germany
Michael Smietana Institut des Biomoleacutecules Max Mousseron CNRS Universiteacute de Montpellier ENSCM France
Zhen‐Yu Tang Department of Pharmaceutical Engineering College of Chemistry and Chemical Engineering Central South University Changsha China
Wouter S Veldmate Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
Anders Vik School of Pharmacy University of Oslo Oslo Norway
Mario Waser Institute of Organic Chemistry Johannes Kepler University Linz Linz Austria
Youwei Xie Max‐Planck‐Institut fuumlr Kohlenforschung Muumllheim Germany
Ian S Young Bristol‐Myers Squibb Company Chemical Development New Brunswick NJ USA
Alexandros L Zografos Department of Chemistry Laboratory of Organic Chemistry Aristotle University of Thessaloniki Thessaloniki Greece
There is pleasure in the pathless woodsthere is rapture in the lonely shore
there is society where none intrudesby the deep sea and music in its roar
I love not Man the less but Nature moreLord Byron
Preface
The first time I came across with the idea of editing a book that merges selected chapters of biosynthesis and total synthesis was when I was teaching postgraduate courses of natural product synthesis at Aristotle University of Thessaloniki This period I realized that the best way to teach youngsters synthesis was to start from the very origin of inspiration nature and its tools biosynthesis
Over the last decades biosynthesis is filling our gaps of understanding the complex mechanisms of nature and pro-vides useful sources of inspiration not only in the way natural products can be synthesized but also by directing synthetic chemists in developing atom‐economical efficient synthetic methods Several are the examples that mimic biosynthetic guidelines from modern iterative alkylations and aldol reactions to CH oxidations that compile nowadays the modern toolbox of organic synthesis
The handed book is constructed in the logic of presenting the parallel development of biosynthesis and organic meth-odology and how these can be applied in efficient syntheses of natural products The book is divided into four sections each representing the four major biosynthetic pathways of natural products namely acetate mevalonate shikimate biosynthetic pathways and the mixed biosynthetic pathways
of alkaloids These sections are divided into chapters that represent selected classes of natural products for example lipids sesquiterpenoids lignans etc Each of these chapters is further divided into three distinct subchapters (a) biosyn-thesis (b) methodological section and (c) application of the described methodology in the total synthesis of the described family of natural products By this way the readers can be focused in the direct comparison between biosyn-theses and the developed methodologies to construct the crucial for each class of natural product carbon bonds Although the book as it develops is focused on presenting the power of biosynthesis and how this power can be applied in providing inspiration for the efficient synthesis of natural products it was not the authors will to present only biomi-metic total syntheses but rather to exploit the modern synthetic methodologies and recognize their disabilities for further improvement
Of course this book will not have been realized without the excellent work of renowned scientists worldwide working either in the field of biosynthesis or total synthesis who collected the existing knowledge on biosynthesis analyzed the existing modern methodologies and presented a bouquet of selected total syntheses Throughout our
xvi PrEfACE
endeavor to complete this book I learned many things from their expertise but I also realized that only with tight collab-orations you can build long‐lasting friendships I would like to thank them all once again for their trust and effort to complete this book We all hope that the current work will contribute to a better understanding of the current status of
organic chemistry and to the discovery of novel strategies and tactics for the synthesis of natural products
Alexandros L ZografosSeptember 2015
Thessaloniki Greece
From Biosynthesis to Total Synthesis Strategies and Tactics for Natural Products First Edition Edited by Alexandros L Zografoscopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
11 FROM PRIMARY TO SECONDARY METABOLISM THE KEY BUILDING BLOCKS
111 Definitions
The primary and secondary metabolisms are traditionally distinguished by their distribution and utility in the living organism network Primary metabolites include carbohyshydrates lipids nucleic acids and proteins (or their amino acid constituents) and are shared by all living organisms on Earth They are transformed by common pathways which are studied by biochemistry (Fig 11) Secondary metabo-lites are structurally diverse compounds usually produced by a limited number of organisms which synthesize them for a special purpose like defense or signaling through specific biosynthetic pathways They are studied by natural product chemistry This distinction is not always so obvious and some compounds can be studied in the context of both primary and secondary metabolisms This is especially true nowadays with the use of genetic and biomolecular tools which tend to make natural product sciences more and more integrative However an important point to remember is that the primary metabolism furnishes key building blocks to the secondary metabolism It would be difficult to describe in detail the full biosynthetic pathshyways in this section We tried to organize the discussion as a vade mecum synthetically gathering information from extremely useful sources which will be cited at the end of this chapter
112 Energy Supply and Carbon Storing at the Early Stage of Metabolisms
The sunlight is essential to life except in some part of the deep oceans It provides energy for plant photosynthesis that splits molecules of water into protons and electrons and releases O
2 (Scheme 11) A proton gradient inside the plant
chloroplasts then drags a transmembrane ATP synthase comshyplex that produces adenosine triphosphate (ATP) while elecshytrons released from water are transferred to the coenzyme reducer nicotinamide adenine dinucleotide phosphate hydride (NADPH) A major function of chloroplasts is to fix CO
2 as a combination to ribulose‐15‐bisphosphate (RuBP)
performed by RuBP carboxylase (rubisco) forming an instable ldquoC
6rdquo β‐ketoacid This is cleaved into two molecules
of 3‐phosphoglycerate (3‐PGA) which is then reduced into 3‐phosphoglyceraldehyde (3‐PGAL a ldquoC
3rdquo triose phosshy
phate) during the Calvin cycle This is one of the major metabolites in the biosynthesis of carbohydrates like glucose and a biochemical mean for storing and retaining carbon atoms in the living cells
113 Glucose as a Starting Material Toward Key Building Blocks of the Secondary Metabolism
Glucose‐6‐phosphate arises from the phosphorylation of glucose It is the starting material of glycolysis an important process of the primary metabolism which consists in eight enzymatic reactions leading to pyruvic acid (PA)
FROM BIOSYNTHESES TO TOTAL SYNTHESES AN INTRODUCTION
Bastien Nay1 and Xu‐Wen Li2
1
1 Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France2 Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
2 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
(Scheme 12) Important intermediates for the secondary metabolism are produced during glycolysis Glucose glucose‐6‐phosphate and fructose‐6‐phosphate can be converted to other hexoses and pentoses that can be oligoshymerized and enter in the composition of heterosides Additionally fructose‐6‐phosphate connects the pentose
phosphate pathway leading to erythrose‐4‐phosphate toward shikimic acid which is a key metabolite in the biosynthesis of aromatic amino acids (phenylalanine tyrosine or C
6C
3
units) and C6C
1 phenolic compounds The next important
intermediate in glycolysis is 3‐PGAL which can be redishyrected toward methylerythritol‐4‐phosphate (mEP) in the
Primary metabolism Secondary metabolism
The field ofbiochemistry
The field ofnatural product
chemistry
Essential toliving organisms
Essential to theproducer organisms
under particularconditions
Nucleic acids (DNARNA) carbohydrates
lipids amino acidspeptides and proteins
Alkaloids terpenespolyketides
polyphenols andtheir heterosidic form
Biological effects(defense signaling)
Biosyntheticpathways
Main compoundclasses
FIGURE 11 Primary versus secondary metabolisms
PS-I
PS-IIendash
hν H2O H+ and O2
ATP synthase
NADP reductase
H+ (gradient)
H+ADP + Pi ATP
NADPHH+NADP+
H+
Thylakoidcompartment
Chloroplaststroma
Thylakoidmembrane
(a) Light dependent process
(b) Light independent process
CO2
RuBP
Rubisco
Unstable β-ketoacid3-PGA 13-diPGA
ATP
ADP3-PGAL
NADPHH+
NADP+
(Calvin cycle)
trioses tetroses pentoses hexoses (eg glucose) heptoses
Chloroplaststroma
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
CH2OPO32ndash CH2OPO3
2ndash CH2OPO32ndash
CH2OPO32ndash
OOH
HO
HOO
HO
HO2CCO2H
HO
CO2PO32ndash
HO
CHOHO
endash
Cytosol
SCHEME 11 The photosynthetic machinery (PS‐I and PS‐II photosystems I and II)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 3
chloroplast mEP is a starting block in the biosynthesis of terpenes through C
5 isoprene units (isopentenyl diphosphate
(IPP) and dimethylallyl diphosphate (DmAPP)) especially those in C
10 C
20 and C
40 terpenes 3‐PGA is a precursor of
serine and other amino acids while phosphoenolpyruvate (PEP) the precursor of PA is also an intermediate toward the previously mentioned shikimic acid Lastly PA is not only a precursor of the fundamental ldquoC
2rdquo acetyl coenzyme A
(AcCoA) unit but also an intermediate toward aliphatic amino acids and mEP
AcCoA is the building block of fatty acids polyketides and mevalonic acid (mVA) a cytosolic precursor of the C
5 isoprene units for the biosynthesis of terpenes in the
C15
and C30
series (mind it is different from the mEP pathway in product and in cell location) Finally AcCoA enters the citric acid or Krebs cycle which leads to several precursors of amino acids These are oxaloacetic acid precursor of aspartic acid through transamination (thus toward lysine as a nitrogenated C
5N linear unit and methishy
onine as a methyl supplier) and 2‐oxoglutaric acid preshycursor of glutamic acid (and subsequent derivatives such as ornithine as a nitrogenated C
4N linear unit) All these
amino acids are key precursors in the biosynthesis of many alkaloids
114 Reactions Involved in the Construction of Secondary Metabolites
most reactions occurring in the living cells are performed by specialized enzymes which have been classified in an intershynational nomenclature defined by an enzyme commission (EC) number There are six classes of enzymes depending on the biochemical reaction they catalyze EC‐1 oxidoreducshytases (catalyzing oxidoreduction reactions) EC‐2 transfershyases (catalyzing the transfer of functional groups) EC‐3 hydrolases (catalyzing hydrolysis) EC‐4 lyases (breaking bonds through another process than hydrolysis or oxidation leading to a new double bond or a new cycle) EC‐5 isomershyases (catalyzing the isomerization of a molecule) and EC‐6 ligases (forming a covalent bond between two molecules) many subclasses of these enzymes have been described depending on the type of atoms and functional groups involved in the reaction and if any on the cofactor used in this reaction For example several cofactors can be used by dehydrogenases like NAD(P)NAD(P)H FADFADH
2 or
FmNFmNH2 For a description of this classification the
reader can refer to specialized Internet websites like ExplorEnz [1] What is important to realize is that most enzymes are substrate specific and have been selected during
CHOHO
CO2H
O
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
HOOH
HO
HOH2CO
OHC
CH2OPO32ndash
OHHO
CO2H
HO
OH
OH
TYR
PHE
CO2H
NH2
TRP
CH2OPO32ndash
HO
HOH2COH
MEP
C10 C20 and C40 terpenes
CO2HHO
SER
GLY CYS
CO2HOPO3
2ndash
CH2OH
OHHO2C
MVA
C15 and C30 terpenes
Polyketides
Krebscycle
(citric acid)
VAL ALA ILE LEU
CO2H
O
HO2C
CO2HO
CO2H
ASP
LYS
GLU
Glucose-6-phosphate
Fructose-6-phosphate
Erythrose-4-phosphate
3-PGAL
OP2O63ndash OP2O6
3ndash
IPP DMAPP
3-PGA PEP PA
O SCoAAcCoA
3x
Cytosol
Cytosol
Chloroplasts
Chloroplasts
Glycolysis
C2
C5
Shikimic acid Oxaloaceticacid
2-Oxoglutaricacid
MET
THR
PRO
ARGORN
Alkaloids Alkaloids Alkaloids Alkaloids
Aminoacids
Peptidesproteins
Phenolics
C1
C5N C4N
ldquoCH3rdquo
(Indole)C2NC6C3C6C2N C6C1
Pentosephosphatepathway
SCHEME 12 The building block chart involving glycolysis and the Krebs cycle
4 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
evolution to perform specific transformations making natural products with often and yet unknown functions
Secondary metabolites arise from specific biosynthetic pathways which use the previously defined building blocks The bunch of organic reactions involved in these biosynshytheses allows the construction of natural product frameshyworks which are finally diversified through ldquodecorationrdquo steps (Scheme 13) It is not the purpose of this introductive chapter to describe in detail all biosynthetic pathways and the reader can refer to excellent books and articles which have been published elsewhere [2 3]
The reactions involved in the construction of natural product skeletons will be described later for representative classes of compounds The identification of the building block footprint in the natural product skeleton will be emphasized as much as possible sometimes referring to biogenetic speculations [4] After the framework construction the decoration steps will involve as diverse reactions as aliphatic CH oxidations (eg involving a cytochrome P
450 oxygenase) occasionally triggering a
rearrangement heteroatom alkylations (eg methylation by S‐adenosylmethionine) or allylation (by DmAPP) esterifications heteroatom or C‐glycosylations (leading to heterosides) radical couplings (especially for phenols) alcohol oxidations or ketone reductions amineketone transaminations alkene dihydroxylations or epoxidations oxidative halogenations BaeyerndashVilliger oxidations and further oxygenation steps At the end of the biosynthesis such transformations may totally hide the primary building block origin of natural products
115 Secondary Metabolisms
1151 Polyketides Polyketides (or polyacetates) are issued from the oligomerization of C
2 acetate units performed
by polyketide synthases (PKS) and leading to (C2)
n linear
intermediates [5 6] If the (C2)
n intermediates arise from
successive Claisen reactions performed by ketosynthase
domains (KS in nonreducing PKS) a highly reactive poly‐ β‐ketoacyl intermediate H(CH
2CO)
nOH is formed
leading to phenolic and aromatic products through further intramolecular Claisen condensations Furthermore highly reducing PKSs are made of specialized enzymatic subunits working in line or iteratively to functionalize each C
2 linker
bond as CH(OH)CH2 (by ketoreductases (KR)) then as
HCCH (by dehydratases (DH)) and as CH2CH
2 (by enoyl
reductases (ER)) leading to a high degree of functionalization of the final product (Fig 12) By these iterative sequences highly reduced polyketides which can be either linear macrocyclized or polycyclized depending on the reactivity of the biosynthetic intermediates can be formed [7] With the same logic fatty acids are also biosynthesized by fatty acid synthases
moreover the PKS enzyme can be hybridized with nonshyribosomal peptide synthetase (NRPS) domains (see also ldquoNRPS metabolites and peptidesrdquo in the ldquoAlkaloidsrdquo secshytion) leading to the acylation of an amino acid by the (C
2)
n
acyl intermediate As previously the functionalization of the acyl chain depends on the PKS enzyme and the PKSNRPS products are also extremely diversified (eg hirsutellone B Fig 12) [8]
1152 Terpenes Terpenes are derived from the oligoshymerization of the C
5 isoprene units DmAPP and IPP Both
precursors are prompt to generate either an allylic cation (the diphosphate is a good leaving group) or a tertiary carbocation respectively which makes the IPP easy to react with DmAPP (Scheme 14) This reaction happens in the active site of a terpene synthase which activates the deparshyture of the diphosphate group from DmAPP thanks to Lewis acid activation (a metal like mg2+ mn2+ or Co2+ is present in the enzyme active site [9]) This leads to geranyl (C
10
monoterpene precursor) or farnesyl (C15
sesquiterpene precursor) diphosphate depending on the location of the enzyme (chloroplast for the mEP pathway or cytosol for the mVA pathway) Geranylgeranyl (C
20 diterpene) and
Constructionreactions
Decoration reaction(functionalization)Building
blocksNatural product
frameworksNatural products
HH
OH OAcH
HO O OH
OHO
OBz
P450
P450
P450
P450
P450 P450 Oxidativeauxiliaryenzymes
Taxadiene
OP2O63ndash
OP2O63ndash
3times
(a)
(b)
10-Deacetylbaccatin III
DMAPP
IPPand electrophilic
cyclizations
SCHEME 13 (a) From building blocks to natural products and (b) the example of 10‐deacetylbaccatin III
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 5
farnesylfarnesyl (C30
triterpene precursor) diphosphates can also be obtained by further additions of IPP leading to longer linear intermediates
The cyclization of linear precursors is achieved by speshycialized cyclases which generate a poorly functionalized natural product framework [10 11] Auxiliary enzymes such as oxygenases then increase the complexity and the diversity
of compounds by further functionalization (Scheme 13b) [12] A high degree of oxidation can be observed in comshypounds like thapsigargin paclitaxel or bilobalide (Fig 13) The biosynthesis of this last compound for example involves a high oxygenation pattern two Wagnerndashmeerwein rearrangements and several oxidative cleavages leading to the loss of five carbons The resulting natural products can
KS
S-ACP
O O
KR
S-ACP
OH O
S-MAT
O
YesNo
R
R
R
YesNoDH
S-ACP
OR
YesNoER
S-ACP
OR
YesNoTE
OH
OR
New cycle
New cycleor
release
New cycleor
release
New cycleor
release
Release
R in
crem
ente
d by
two
carb
ons
HO
O
S-ACP
O
Claisen condensation (ndashCO2)
reduction (NADPH)
dehydration (ndashH2O)
+
reduction (NADPH)
hydrolysis (H2O)
O O NH
OH
OH
H H
H H
Hirsutellone B(mixed PKSNRPS
product)
OO
O
OHOH
HO OH
Norsolorinic acid
O
O
O
O CHO
OH
HH
H
H
HH
OHHemibrevetoxin B
OOHO
OHO
HOHO OH
Erythronolide A
OH
OH
HO
Panaxytriol
From ahexanoylstartingblock
H
H
Fromtyrosine
H
1C lost fromdecarboxylation
FIGURE 12 Chemical logic of polyketide construction leading to variable functionalization of the elongated acyl chain and examples of resulting chemical diversity ACP acyl carrier protein DH dehydratase ER enoyl reductase KR ketoreductase KS ketosynthase mAT malonyl acyl transferase TE thioesterase
DMAPPOP2O6
3ndashOP2O6
3ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
Geranyl diphosphate (GPP)2 times IPP
H H
Geranylgeranyl diphosphate (GGPP)
Examplesverticillane
casbanetaxane
phorbol
Exampleslabdane
pimaranekauraneabietane
aphidicolanegibberellane
IPP
SCHEME 14 Early assembly of C5 units in terpene biosynthesis leading to diterpenes (C
20)
6 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
thus be extremely modified with structures whose biogenetic origin is far from being obvious at first sight and cannot be determined without further experiments such as isotopic labeling
1153 Flavonoids Resveratrols Gallic Acids and Further Polyphenolics We have previously discussed the polyketide origin of some phenolic compounds based on the (C
2)
n motif Other polyphenols like gallic acids are directly
derived by the aromatization of shikimic acid (C6C
1 building
block Scheme 12) [13] The C6C
3 building blocks are availshy
able from the conversion of phenylalanine and tyrosine into cinnamic and p‐coumaric acids respectively and then by further hydroxylation steps (Scheme 15) These can dimerize into lignans (eg podophyllotoxin) [14 15] through radical processes or converted to low molecular weight compounds like eugenol coumarins or vanillin [16] The coenzyme A thioesters of these C
6C
3 acids can be used as initiator units
by specialized ketosynthases for an elongation by two acetyl units leading to aromatic polyketides like styrylpyrones or diarylheptanoids (eg curcumin) [17] Important comshypounds from this metabolism are flavonoids (C
6C
3C
6) [18]
and stilbenoids (C6C
2C
6) (a decarboxylation occurs during
the aryl cyclization) [19] which are synthesized by chalcone synthase and stilbene synthase respectively Flavonoids (eg catechin) and stilbenes (eg resveratrol) are present in large amounts in fruits and vegetables and may exert their radical scavenging properties in vivo
1154 Alkaloids Alkaloids are nitrogen‐containing compounds The nitrogen(s) can be involved in an amine function conferring basicity to the natural product (like ldquoalkalirdquo) or in less or nonbasic functions such as an amide a nitrile an isonitrile or an ammonium salt (quaternary amines) For amines protonation often occurs at physiological pH and may condition their biological activity In many cases the nitrogen is biogenetically derived from an amino acid We will thus discuss alkaloids according to their amino acid origin
Alkaloids Derived from the Krebs Cycle (Lysine and Ornithine Derived) As shown previously (Scheme 12) the Krebs cycle is a crucial metabolic process which leads to α‐ketoacids (oxaloacetic and 2‐oxoglutaric acids) Their enzymatic transamination affords the two amino acidsmdashaspartic acid and glutamic acidmdashwhich are the direct biosynthetic precursors of amino acids lysine and ornithine respectively These in turn produce cadaverine a ldquoC
5Nrdquo unit
and putrescine a ldquoC4Nrdquo unit which are major components
for the biosynthesis of important alkaloids as will be discussed later (Scheme 16) Additionally ornithine is a precursor of arginine another important amino acid
ornithine‐derived alkaloids (incorporating the c4n
unit) Putrescine is derived from the decarboxylation of ornithine and is a precursor of linear polyamines like spermshyine After enzymatic methylation of one amine of putrescine
C10 C15
C15
C10
C10
C30
CO2H
Chrysanthemic acid
OH
Menthol
O
HO
H
HOGlc
MeO2CLoganin (iridoid)
H
H
H
HH
OO
OParthenolide
O
O
OHOHO
OAcHO
OnC3H7
O
O
O
nC7H15
Thapsigargin
O
O
OO
H
H
O
Cleavage
H
H
Artemisinin
C20 C40
O
O
OO
OO
OHOHH
Bilobalide(highly rearrangedand missing 5 C)
CleavageH
Highoxidativecleavages
HO OAcH
AcO O OH
OOOBz
O
Ph
BzNH
OHFrom PHE Paclitaxel
C25O
H
HO
OHCH
OH Ophiobolin A
H
HO
H
H
HHCholesterol
(missing 3 C WM)
H
H H H
H
H
Hopene
β-Carotene
C30 - 3
C15
C20 - 5WM
WM
FIGURE 13 Chemical diversity in the terpene series (Wm Wagnerndashmeerwein shifts lost carbons bold bonds are remnant of primary building blocks)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 7
in the presence of S‐adenosylmethionine transamination of the other affords γ‐(N‐methylamino)aldehyde [20] The resulting cyclic iminium is a key intermediate in the formation of many medicinally important alkaloids such as
the plant‐derived compounds cocaine atropine or the calysshytegines [21 22] Indeed this iminium is a mannich acceptor which can react with various nucleophiles the first of those being the carbanion of acetyl‐CoA Thus after a stepwise
CO2H
RCinnamic acid (R=H)p-Coumaric acid (R=OH)
RO
PHETYR
OH
[H] [O]
lignans
OH
O
O
O
O
OMeMeO
OMe
C mdash C andor C mdash Oradical couplings
After E Zisomerization
for example Podophyllotoxin
O ORO
Coumarins(eg scopoletin)
[O]
Prenylationcyclizationcleavage[O]
O OO
Furocoumarins(eg psoralen)
RO
nAcetyl-CoA
thencyclization
n = 2 styrylpyrones
O O
OMe
n = 3 avonoids(from chalcone synthase)
C6C3
H
H H
From AcCoA
O
n = 3 stilbenoids(from stilbene synthase)
HO
OH
OHFrom AcCoA From AcCoA(ndashCO2)
Resveratrol
HO
OH
OH
OH
OH
H
Catechin
MeO Yangonin
From AcCoA
SCHEME 15 The phenylpropanoid biosynthetic pathways
LYS
CO2
Cadaverine
O NH
H2O
NH
Piperideineiminium
Pelletierine
AcAcCoA
CO2
NH
O
N
O
Pseudopelletierine
NH
Ph
OH
Ph
O
Lobeline
NH
δ+
δndash
N
H
H
N
H
OH
Lupinine
N
H N
H
(+)-Sparteine
N
NH
O (ndash)-Cytisine
NH
CO2H
Pipecolic acid
N
OHHO
OH
HO
H
Castanos permine
ORN
CO2
NH2 NH2 NH2 NH2
NH2
NH2
NH2
PutrescineO
NHMe
N-Methylpyrrolinium
2 timesAcCoA
NMe
ARG
n = 1 spermidinen = 2 homospermidine
NMe
O
HygrineCO2
CO2
MeN
O
[O]
TropinoneCO2
HNHO
OHOH
OH
Calystegine B2
MeN
OAtropine
O
Ph
HO
NMe
NMe
NMe
O
Cuscohygrine
HN
HN
NH2
( )n
O
n = 2
N
HHO
Retronecine
HH
H
H
H
HO
H H
H
SCHEME 16 Lysine‐ and ornithine‐derived alkaloid biosynthetic pathways (mind the structural similarities)
8 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
elongation by two AcCoA units either decarboxylation can occur leading to the acetonylpyrrolidine hygrine or a secshyond mannich reaction by the intramolecular attack of the acetoacetate anion onto an oxidation‐derived pyrrolinium leading to the tropane skeleton (tropinone) The acetoacetyl‐CoA intermediate can also react intermolecularly with another pyrrolinium cation leading to cuscohygrine after decarboxylation Finally the pyrrolizidine alkaloids [23] are derived from homospermidine which when submitted to terminal oxidative deamination leads to the bicyclic skelshyeton of retronecine and further Senecio alkaloids We can mention herein that ornithine is a biosynthetic precursor of arginine bearing a guanidine function which is an intermediate toward the toxic compounds tetrodotoxin and saxitoxin (not shown)
lysine‐derived alkaloids (incorporating the c5n
unit) From lysine to piperidine alkaloids the biosynthetic steps parallel the one previously described from ornithine Indeed the oxidative deamination of cadaverine affords a δ‐amino aldehyde which cyclizes through imine formation into piperideine Protonation results in a mannich acceptor which is able to react with various nucleophiles such as β‐ketothioester anions The first product of these reactions is pelletierine which can further react through an intramolecular mannich
reaction leading to pseudopelletierine Quinolizidines [24] can also be formed first from the mannich reaction of the piperideine acceptor with the corresponding enamine nucleoshyphile and then after additional transformation steps leading for example to lupinine sparteine or cytisine
Indolizidine alkaloids [15] such as castanospermine and swainsonine are formed from pipecolic acid an amino acid derived from lysine which can be elongated by malonyl‐CoA followed by ring closure When protonated these alkaloids are oxonium mimics strongly inhibiting glycosidases
Tyrosine‐ and Phenylalanine‐Derived Alkaloids Tyrosine and phenylalanine amino acids are bearing the phenylshyethylamine moiety of many medicinally relevant alkaloids Further hydroxylations on the aromatic carbocycle or on the aliphatic part can be observed methylations can occur on phenolic oxygens and on the amine leading to catecholamines (adrenaline noradrenaline dopamine) Arylethylamines are also usual to react with endogenous aldehydes through PictetndashSpengler reactions [25] leading to important biosynthetic intermediates (Scheme 17) like
bull Reticuline from the reaction with 4‐hydroxyphenylacetshyaldehyde toward benzyltetrahydroisoquinoline alkaloids
NH2
Phenylethylamines
RONH TetrahydroisoquinolinesRO
RʹCHO
Rʹ
NH
(CH2)n
MeO
HO
Benzyltetrahydroisoquinoline(n = 1) for example reticuline
orPhenethyltetrahydroisoquinoline
(n = 2) eg autumnaline
IntermolecularCmdashO phenol
couplings
Curarealkaloids
IntramolecularC mdash C phenol
couplings
ONMe
HHO
H
HO
Morphine
NMe
MeO
HO
MeOOH
Isoboldine
O
O
OMe
NO2
CO2H
Aristolochic acid
OMe
O
NHAcMeO
MeOMeO
Colchicine
N
O
O
OMe
OMeBerberine
IntramolecularCmdash C coupling
and furtheroxidative events
Rʹ derivedfrom PHE
Pictet-Spenglerreaction
TYR
HO
HO CHO
HNHO
HO
OH
OMeO
OH
NMe
[O]
GalantamineNorbelladine
Rʹ derived fromsecologanin
NAc
HO
HO
O
CO2MeH
H
H
OHIpecosideaglycone
Ipecac alkaloids
1ClostCmdash C cleavage
and ring extension
H
ROH
1C lost
Cleavage
SCHEME 17 Tyrosine‐derived alkaloid biosynthetic pathways (double head arrows figure bond cleavages during biosynthetic processes)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 9
morphine berberine tubocurarine isoboldine or the highly modified aristolochic acid [26 27]
bull Automnaline from the reaction with 3‐(4‐hydroxyphenyl)propanal toward phenylethyltetrahydroisoquinoline alkaloids colchicine cephalotaxine or schelhammerishycine [28 29]
bull Ipecoside from the reaction of dopamine with secoloshyganin toward terpene tetrahydroisoquinoline alkaloids ipecoside or emetine
Lastly norbelladine (top of Scheme 17) is issued from the reductive amination of 34‐dihydroxybenzaldehyde (derived from phenylalanine) with tyramine (derived from tyrosine) and constitutes a biosynthetic node leading to Amaryllidaceae alkaloids such as galantamine crinine or lycorine depending on the topology of phenolic couplings In all these biosynthetic routes radical phenolic couplings are key reactions for CC and CO bond formations and rearrangements [30 31]
Tryptophan‐Derived Indole and Indole Monoterpene Alkaloids As for alkaloids derived from tyrosine and phenylalanine those derived from tryptophan are formed after decarboxylation of the amino acid (into tryptamine) and possible hydroxylation of the aromatic carbocycle (eg serotonin) and N‐methylation (eg psilocin) As previously tryptamine can also react through PictetndashSpengler reactions to form tetrahydro‐β‐carbolines which can be aromatized for example into harmine (Scheme 18) [16]
When the aldehyde partner of the PictetndashSpengler reacshytion with tryptamine is the terpene secologanin strictosidine is formed as an entry toward the vast monoterpene indole alkaloids [32 33] Hydrolysis of the glucosidic part releases the strictosidine aglycone bearing an aldehyde while iminshyium formation and further cyclization and reduction can lead to ajmalicine (from oxocyclization) or yohimbine (from carshybocyclization) These alkaloids are referred to as from the Corynanthe type with the monoterpene carbon skeleton unmodified Although it misses one carbon and has a very
RO
Carbonylcompounds
TRPNH
NH2
Indolethylamines(eg serotonine)
RONH
NH
Rʹ NH
N
MeOPictet-Spenglerreaction for example Harmineβ-Carbolines
NH
NHH
O
MeO2C
MeO2C
OH
H
H
Rʹ derived from secologanin
Strictosidine aglycone
NH
N
OH
H
H
Ajmalicine
N
N
O
O
H
H
H
Strychnine (C lost)Corynanthe type
Aspidosperma type Iboga type
N
N
OH
OMe
Quinine (C lost)
N
NH CO2Me
Catharanthine
NH
N
CO2MeH
OAc
OH
Vindoline
Complextransformations
NH
NMe
HN
MeN
H
H
Chimonanthine
Cyclizationdimerization
Prenylationcyclization
NH
NMeHO2C
H
Lysergic acid
H
H
Ergot alkaloidsPyrroloindoles Indole monoterpene
1C lost
1C lostFrom AcCoA
SCHEME 18 Tryptophan‐derived alkaloid biosynthetic pathways (gray parts monoterpenic units)
10 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
different structure strychnine is related to the Corynanthe alkaloids incorporating two carbons from acetyl‐CoA Highly modified monoterpene skeletons are derived from the Corynanthe core through CC bond breaking and reorshyganization leading to Iboga‐type (eg catharanthine) and Aspidosperma‐type (eg vindoline) alkaloids The antishycancer drug vinblastine is a heterodimer resulting from the nucleophilic attack of vindoline on a mannich acceptor resulting from catharanthine found in madagascar perishywinkle (Catharanthus roseus) The heteroaromatic comshypounds ellipticine camptothecin and quinine are also derived from a Corynanthe‐type precursor although in this case the biosynthetic relationship may not be obvious due to deep modifications of the skeleton
Finally two important classes of compounds have to be mentioned since they have inspired many synthetic chemists The pyrroloindole alkaloids result from the cyclization of tryptamine as found in physostigmine (formed by a cationic mechanism after methylation in position 3 of the indole not shown) or in chimonanthine (presumably formed by a radical coupling mechanism Scheme 18) The ergot alkaloids are derived from the 33‐dimethylallylation on position 4 of the indole in trypshytophan whose further cyclization and oxidation processes afford the natural products (eg lysergic acid Scheme 18 and ergotamine) which have had important medical applications [34]
NRPS Metabolites and Peptides NRPS enzymes assemble amino acids including nonproteinogenic ones into oligopeptides The enzymes contain several modules and especially an adenylation domain (A) which specifically selects and activates the amino acid to be transferred as a thioester on the nearby peptidyl carrier protein (PCP) [2] A condensation module (C) then catalyzes the formation of the peptide bonds between the newly introduced amino acyl‐PCP (bearing a free amine) and the elongated peptidyl‐PCP thioester At the end of the elongation a cyclization can occur into cyclopeptides but the peptide can also be
transferred to auxiliary enzymes like methyltransferases glycosyltransferases or oxidases (vancomycins are typical products of such functionalizations) [35 36] The formation of heterocycles is also frequently encountered in this metabolism as in penicillins that are derived from the tripeptide α‐aminoadipoyl‐cysteinyl‐valine or telomestatin (Fig 14) [2]
Other Alkaloid Origins There are many other nitrogen sources involved in alkaloid biosyntheses for example nicotinic acid (originated from aspartic acid and intershymediate in nicotine and anabasine biosyntheses) and anthranilic acid (originated from tryptophan and intermedishyate toward acridines or aurachins) The amination reaction (eg through transamination of carbonyl compounds) is also a way to introduce nitrogens in natural products for example from fatty acids steroids (toward Solanum alkaloids or cyclopamines) or other terpenoids (aconitine and atisine have diterpene skeletons while Daphniphyllum alkaloids are triterpene derivatives) Finally nucleic acids can also be precursors of alkaloids like the well‐known caffeine
12 FROM BIOSYNTHESIS TO TOTAL SYNTHESIS STRATEGIES TOWARD THE NATURAL PRODUCT CHEMICAL SPACE
121 The Chemical Space of Natural Products
Natural products occupy an important place in human com munities as demonstrated by their vast use from ancient times to nowadays like dyes fibers oils pershyfumes agrochemicals or drugs Broadly both primary and secondary metabolites could be classified as natural products while the latter as discussed previously are usushyally regarded as the ldquonatural productsrdquo owing to their comshyplexity and diversity arising from a variety of biosynthetic pathways The structural chemical diversity found among
N
S
CO2HO
HNPhH2C
O
Penicillin G
OH
HN
HN
O
O
ONH2
O
OHO
NHMe
OCl
O
NH
NH
NH
O
ClHO
O
HN
H
H
HOOH
OHHO2C
Vancomycin aglycone
ON
NO
O
NN
O
O
N
N O
Me
S
N N
OMeH
Telomestatin
L-AAA-L-CYS-D-VAL
Enzymes
FIGURE 14 Structural diversity of nonribosomal peptide compounds (AAA α‐aminoadipic acid)
FROm BIOSyNTHESIS TO TOTAL SyNTHESIS STRATEGIES TOWARD THE NATuRAL PRODuCT CHEmICAL SPACE 11
all living organisms defining the chemical space of natural products [37] is the consequence of their evolution occurshyring as an adaptation of organisms to their environment It is commonly believed that secondary metabolites are proshyduced as messengers by living organisms or as weapons against enemies and thus they should have certain biological activities in a medicinal point of view [38] Indeed natural products are regarded as one of the main sources of medicines (Fig 15) From the traditional medicinal extracts to every single bioactive molecule the methods of extraction purification identification and biological investigation of natural products have been well established Their complex structures and interesting properties have attracted synthetic chemists to accomplish their total syntheses and that of medicinally relevant anashylogs sometimes in the industrial context [39] Thus tarshygeting the chemical space of natural products has never been more relevant than today Although the discovery of natural products demands time and labor‐consuming manipulations it is worth to notice that the knowledge on this chemical space is still continually growing while biological advances allow for discovering and undershystanding potential targets However increasing the chemical space of human‐made compounds based on natural products should benefit from transdisciplinary colshylaborations such as the use of coupled biosynthetic and chemical synthetic methods to design original ldquounnatural natural productsrdquo [40]
122 The Biosynthetic Pathways as an Inspiration for Synthetic Challenges
1221 Precursor‐Directed Biosyntheses and Mutashysynthetic Strategies to Increase the Chemical Space of Natural Products As the genetics and biochemistry of natural product biosynthesis are better understood novel biosynthetic techniques have been developed to study and generate new diversity in natural product analogs (Scheme 19) Precursor‐directed biosynthesis (PDB) is considered as the earliest example of combining chemical and biological methods for the generation of complex natural product analogs [41 42] This approach compared with the biosynthetic pathway of wild‐type metabolites (Scheme 19a) involves the feeding of analogs of the natural biosynthetic building blocks to the living organisms (Scheme 19b) usually bacteria or fungi which incorporate the modified precursors into the biosynthesized compound mutasynthesis also termed as mutational biosynthesis (mBS) involves the inactivation of a key step of the bioshysynthesis in a mutant microorganism (Scheme 19c) which can then be fed by various modified or advanced building blocks (mutasynthons Scheme 19d) [43] These mutasshyynthons could not be incorporated by the wild type due to specificities of the enzymatic machinery Build up on PDB mBS eliminates the natural biosynthetic intermediate thus generating a less complex mixture of metabolites and making the purification or yield of target products better
O NH
OHO
O
OO
O
O
OH
HOO
O O
OH
HOO
NH ONH2
O
ONH
NH
O
OCl Cl
O
O
OH
HN
HN
HN
HN
OO
HO
HN
OH
OHOH
O
O
HO
HO
OHO
O
OHH2N
O
OH3C
H
O
H
CH3
CH3
H
O O
NO
H
O
HO
O
NO
O
NH
HN
OHO
HONH
ON
H
HO
O
H2N
O OH
ONH
OH
ON OHO
OH
HO OS OH
OOOH
OHO
O
ON
OO
O
HO
O
O OH
OO
Micafunginantifungal
Rapamycinimmunosuppressive
Galantamineanti-Alzheimer
Taxolanticancer
Artemisininantimalarial
Vancomycinantibiotic
FIGURE 15 Some famous natural products currently used as drugs
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product
xiv LIST OF CONTRIBUTORS
Franccediloise Schaefers Department of Chemistry and Center for Integrated Protein Science Munich (CIPSM) Biosystems Chemistry Technische Universitaumlt Muumlnchen Munich Germany
Michael Smietana Institut des Biomoleacutecules Max Mousseron CNRS Universiteacute de Montpellier ENSCM France
Zhen‐Yu Tang Department of Pharmaceutical Engineering College of Chemistry and Chemical Engineering Central South University Changsha China
Wouter S Veldmate Institute for Molecules and Materials Radboud University Nijmegen Nijmegen The Netherlands
Anders Vik School of Pharmacy University of Oslo Oslo Norway
Mario Waser Institute of Organic Chemistry Johannes Kepler University Linz Linz Austria
Youwei Xie Max‐Planck‐Institut fuumlr Kohlenforschung Muumllheim Germany
Ian S Young Bristol‐Myers Squibb Company Chemical Development New Brunswick NJ USA
Alexandros L Zografos Department of Chemistry Laboratory of Organic Chemistry Aristotle University of Thessaloniki Thessaloniki Greece
There is pleasure in the pathless woodsthere is rapture in the lonely shore
there is society where none intrudesby the deep sea and music in its roar
I love not Man the less but Nature moreLord Byron
Preface
The first time I came across with the idea of editing a book that merges selected chapters of biosynthesis and total synthesis was when I was teaching postgraduate courses of natural product synthesis at Aristotle University of Thessaloniki This period I realized that the best way to teach youngsters synthesis was to start from the very origin of inspiration nature and its tools biosynthesis
Over the last decades biosynthesis is filling our gaps of understanding the complex mechanisms of nature and pro-vides useful sources of inspiration not only in the way natural products can be synthesized but also by directing synthetic chemists in developing atom‐economical efficient synthetic methods Several are the examples that mimic biosynthetic guidelines from modern iterative alkylations and aldol reactions to CH oxidations that compile nowadays the modern toolbox of organic synthesis
The handed book is constructed in the logic of presenting the parallel development of biosynthesis and organic meth-odology and how these can be applied in efficient syntheses of natural products The book is divided into four sections each representing the four major biosynthetic pathways of natural products namely acetate mevalonate shikimate biosynthetic pathways and the mixed biosynthetic pathways
of alkaloids These sections are divided into chapters that represent selected classes of natural products for example lipids sesquiterpenoids lignans etc Each of these chapters is further divided into three distinct subchapters (a) biosyn-thesis (b) methodological section and (c) application of the described methodology in the total synthesis of the described family of natural products By this way the readers can be focused in the direct comparison between biosyn-theses and the developed methodologies to construct the crucial for each class of natural product carbon bonds Although the book as it develops is focused on presenting the power of biosynthesis and how this power can be applied in providing inspiration for the efficient synthesis of natural products it was not the authors will to present only biomi-metic total syntheses but rather to exploit the modern synthetic methodologies and recognize their disabilities for further improvement
Of course this book will not have been realized without the excellent work of renowned scientists worldwide working either in the field of biosynthesis or total synthesis who collected the existing knowledge on biosynthesis analyzed the existing modern methodologies and presented a bouquet of selected total syntheses Throughout our
xvi PrEfACE
endeavor to complete this book I learned many things from their expertise but I also realized that only with tight collab-orations you can build long‐lasting friendships I would like to thank them all once again for their trust and effort to complete this book We all hope that the current work will contribute to a better understanding of the current status of
organic chemistry and to the discovery of novel strategies and tactics for the synthesis of natural products
Alexandros L ZografosSeptember 2015
Thessaloniki Greece
From Biosynthesis to Total Synthesis Strategies and Tactics for Natural Products First Edition Edited by Alexandros L Zografoscopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
11 FROM PRIMARY TO SECONDARY METABOLISM THE KEY BUILDING BLOCKS
111 Definitions
The primary and secondary metabolisms are traditionally distinguished by their distribution and utility in the living organism network Primary metabolites include carbohyshydrates lipids nucleic acids and proteins (or their amino acid constituents) and are shared by all living organisms on Earth They are transformed by common pathways which are studied by biochemistry (Fig 11) Secondary metabo-lites are structurally diverse compounds usually produced by a limited number of organisms which synthesize them for a special purpose like defense or signaling through specific biosynthetic pathways They are studied by natural product chemistry This distinction is not always so obvious and some compounds can be studied in the context of both primary and secondary metabolisms This is especially true nowadays with the use of genetic and biomolecular tools which tend to make natural product sciences more and more integrative However an important point to remember is that the primary metabolism furnishes key building blocks to the secondary metabolism It would be difficult to describe in detail the full biosynthetic pathshyways in this section We tried to organize the discussion as a vade mecum synthetically gathering information from extremely useful sources which will be cited at the end of this chapter
112 Energy Supply and Carbon Storing at the Early Stage of Metabolisms
The sunlight is essential to life except in some part of the deep oceans It provides energy for plant photosynthesis that splits molecules of water into protons and electrons and releases O
2 (Scheme 11) A proton gradient inside the plant
chloroplasts then drags a transmembrane ATP synthase comshyplex that produces adenosine triphosphate (ATP) while elecshytrons released from water are transferred to the coenzyme reducer nicotinamide adenine dinucleotide phosphate hydride (NADPH) A major function of chloroplasts is to fix CO
2 as a combination to ribulose‐15‐bisphosphate (RuBP)
performed by RuBP carboxylase (rubisco) forming an instable ldquoC
6rdquo β‐ketoacid This is cleaved into two molecules
of 3‐phosphoglycerate (3‐PGA) which is then reduced into 3‐phosphoglyceraldehyde (3‐PGAL a ldquoC
3rdquo triose phosshy
phate) during the Calvin cycle This is one of the major metabolites in the biosynthesis of carbohydrates like glucose and a biochemical mean for storing and retaining carbon atoms in the living cells
113 Glucose as a Starting Material Toward Key Building Blocks of the Secondary Metabolism
Glucose‐6‐phosphate arises from the phosphorylation of glucose It is the starting material of glycolysis an important process of the primary metabolism which consists in eight enzymatic reactions leading to pyruvic acid (PA)
FROM BIOSYNTHESES TO TOTAL SYNTHESES AN INTRODUCTION
Bastien Nay1 and Xu‐Wen Li2
1
1 Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France2 Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
2 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
(Scheme 12) Important intermediates for the secondary metabolism are produced during glycolysis Glucose glucose‐6‐phosphate and fructose‐6‐phosphate can be converted to other hexoses and pentoses that can be oligoshymerized and enter in the composition of heterosides Additionally fructose‐6‐phosphate connects the pentose
phosphate pathway leading to erythrose‐4‐phosphate toward shikimic acid which is a key metabolite in the biosynthesis of aromatic amino acids (phenylalanine tyrosine or C
6C
3
units) and C6C
1 phenolic compounds The next important
intermediate in glycolysis is 3‐PGAL which can be redishyrected toward methylerythritol‐4‐phosphate (mEP) in the
Primary metabolism Secondary metabolism
The field ofbiochemistry
The field ofnatural product
chemistry
Essential toliving organisms
Essential to theproducer organisms
under particularconditions
Nucleic acids (DNARNA) carbohydrates
lipids amino acidspeptides and proteins
Alkaloids terpenespolyketides
polyphenols andtheir heterosidic form
Biological effects(defense signaling)
Biosyntheticpathways
Main compoundclasses
FIGURE 11 Primary versus secondary metabolisms
PS-I
PS-IIendash
hν H2O H+ and O2
ATP synthase
NADP reductase
H+ (gradient)
H+ADP + Pi ATP
NADPHH+NADP+
H+
Thylakoidcompartment
Chloroplaststroma
Thylakoidmembrane
(a) Light dependent process
(b) Light independent process
CO2
RuBP
Rubisco
Unstable β-ketoacid3-PGA 13-diPGA
ATP
ADP3-PGAL
NADPHH+
NADP+
(Calvin cycle)
trioses tetroses pentoses hexoses (eg glucose) heptoses
Chloroplaststroma
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
CH2OPO32ndash CH2OPO3
2ndash CH2OPO32ndash
CH2OPO32ndash
OOH
HO
HOO
HO
HO2CCO2H
HO
CO2PO32ndash
HO
CHOHO
endash
Cytosol
SCHEME 11 The photosynthetic machinery (PS‐I and PS‐II photosystems I and II)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 3
chloroplast mEP is a starting block in the biosynthesis of terpenes through C
5 isoprene units (isopentenyl diphosphate
(IPP) and dimethylallyl diphosphate (DmAPP)) especially those in C
10 C
20 and C
40 terpenes 3‐PGA is a precursor of
serine and other amino acids while phosphoenolpyruvate (PEP) the precursor of PA is also an intermediate toward the previously mentioned shikimic acid Lastly PA is not only a precursor of the fundamental ldquoC
2rdquo acetyl coenzyme A
(AcCoA) unit but also an intermediate toward aliphatic amino acids and mEP
AcCoA is the building block of fatty acids polyketides and mevalonic acid (mVA) a cytosolic precursor of the C
5 isoprene units for the biosynthesis of terpenes in the
C15
and C30
series (mind it is different from the mEP pathway in product and in cell location) Finally AcCoA enters the citric acid or Krebs cycle which leads to several precursors of amino acids These are oxaloacetic acid precursor of aspartic acid through transamination (thus toward lysine as a nitrogenated C
5N linear unit and methishy
onine as a methyl supplier) and 2‐oxoglutaric acid preshycursor of glutamic acid (and subsequent derivatives such as ornithine as a nitrogenated C
4N linear unit) All these
amino acids are key precursors in the biosynthesis of many alkaloids
114 Reactions Involved in the Construction of Secondary Metabolites
most reactions occurring in the living cells are performed by specialized enzymes which have been classified in an intershynational nomenclature defined by an enzyme commission (EC) number There are six classes of enzymes depending on the biochemical reaction they catalyze EC‐1 oxidoreducshytases (catalyzing oxidoreduction reactions) EC‐2 transfershyases (catalyzing the transfer of functional groups) EC‐3 hydrolases (catalyzing hydrolysis) EC‐4 lyases (breaking bonds through another process than hydrolysis or oxidation leading to a new double bond or a new cycle) EC‐5 isomershyases (catalyzing the isomerization of a molecule) and EC‐6 ligases (forming a covalent bond between two molecules) many subclasses of these enzymes have been described depending on the type of atoms and functional groups involved in the reaction and if any on the cofactor used in this reaction For example several cofactors can be used by dehydrogenases like NAD(P)NAD(P)H FADFADH
2 or
FmNFmNH2 For a description of this classification the
reader can refer to specialized Internet websites like ExplorEnz [1] What is important to realize is that most enzymes are substrate specific and have been selected during
CHOHO
CO2H
O
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
HOOH
HO
HOH2CO
OHC
CH2OPO32ndash
OHHO
CO2H
HO
OH
OH
TYR
PHE
CO2H
NH2
TRP
CH2OPO32ndash
HO
HOH2COH
MEP
C10 C20 and C40 terpenes
CO2HHO
SER
GLY CYS
CO2HOPO3
2ndash
CH2OH
OHHO2C
MVA
C15 and C30 terpenes
Polyketides
Krebscycle
(citric acid)
VAL ALA ILE LEU
CO2H
O
HO2C
CO2HO
CO2H
ASP
LYS
GLU
Glucose-6-phosphate
Fructose-6-phosphate
Erythrose-4-phosphate
3-PGAL
OP2O63ndash OP2O6
3ndash
IPP DMAPP
3-PGA PEP PA
O SCoAAcCoA
3x
Cytosol
Cytosol
Chloroplasts
Chloroplasts
Glycolysis
C2
C5
Shikimic acid Oxaloaceticacid
2-Oxoglutaricacid
MET
THR
PRO
ARGORN
Alkaloids Alkaloids Alkaloids Alkaloids
Aminoacids
Peptidesproteins
Phenolics
C1
C5N C4N
ldquoCH3rdquo
(Indole)C2NC6C3C6C2N C6C1
Pentosephosphatepathway
SCHEME 12 The building block chart involving glycolysis and the Krebs cycle
4 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
evolution to perform specific transformations making natural products with often and yet unknown functions
Secondary metabolites arise from specific biosynthetic pathways which use the previously defined building blocks The bunch of organic reactions involved in these biosynshytheses allows the construction of natural product frameshyworks which are finally diversified through ldquodecorationrdquo steps (Scheme 13) It is not the purpose of this introductive chapter to describe in detail all biosynthetic pathways and the reader can refer to excellent books and articles which have been published elsewhere [2 3]
The reactions involved in the construction of natural product skeletons will be described later for representative classes of compounds The identification of the building block footprint in the natural product skeleton will be emphasized as much as possible sometimes referring to biogenetic speculations [4] After the framework construction the decoration steps will involve as diverse reactions as aliphatic CH oxidations (eg involving a cytochrome P
450 oxygenase) occasionally triggering a
rearrangement heteroatom alkylations (eg methylation by S‐adenosylmethionine) or allylation (by DmAPP) esterifications heteroatom or C‐glycosylations (leading to heterosides) radical couplings (especially for phenols) alcohol oxidations or ketone reductions amineketone transaminations alkene dihydroxylations or epoxidations oxidative halogenations BaeyerndashVilliger oxidations and further oxygenation steps At the end of the biosynthesis such transformations may totally hide the primary building block origin of natural products
115 Secondary Metabolisms
1151 Polyketides Polyketides (or polyacetates) are issued from the oligomerization of C
2 acetate units performed
by polyketide synthases (PKS) and leading to (C2)
n linear
intermediates [5 6] If the (C2)
n intermediates arise from
successive Claisen reactions performed by ketosynthase
domains (KS in nonreducing PKS) a highly reactive poly‐ β‐ketoacyl intermediate H(CH
2CO)
nOH is formed
leading to phenolic and aromatic products through further intramolecular Claisen condensations Furthermore highly reducing PKSs are made of specialized enzymatic subunits working in line or iteratively to functionalize each C
2 linker
bond as CH(OH)CH2 (by ketoreductases (KR)) then as
HCCH (by dehydratases (DH)) and as CH2CH
2 (by enoyl
reductases (ER)) leading to a high degree of functionalization of the final product (Fig 12) By these iterative sequences highly reduced polyketides which can be either linear macrocyclized or polycyclized depending on the reactivity of the biosynthetic intermediates can be formed [7] With the same logic fatty acids are also biosynthesized by fatty acid synthases
moreover the PKS enzyme can be hybridized with nonshyribosomal peptide synthetase (NRPS) domains (see also ldquoNRPS metabolites and peptidesrdquo in the ldquoAlkaloidsrdquo secshytion) leading to the acylation of an amino acid by the (C
2)
n
acyl intermediate As previously the functionalization of the acyl chain depends on the PKS enzyme and the PKSNRPS products are also extremely diversified (eg hirsutellone B Fig 12) [8]
1152 Terpenes Terpenes are derived from the oligoshymerization of the C
5 isoprene units DmAPP and IPP Both
precursors are prompt to generate either an allylic cation (the diphosphate is a good leaving group) or a tertiary carbocation respectively which makes the IPP easy to react with DmAPP (Scheme 14) This reaction happens in the active site of a terpene synthase which activates the deparshyture of the diphosphate group from DmAPP thanks to Lewis acid activation (a metal like mg2+ mn2+ or Co2+ is present in the enzyme active site [9]) This leads to geranyl (C
10
monoterpene precursor) or farnesyl (C15
sesquiterpene precursor) diphosphate depending on the location of the enzyme (chloroplast for the mEP pathway or cytosol for the mVA pathway) Geranylgeranyl (C
20 diterpene) and
Constructionreactions
Decoration reaction(functionalization)Building
blocksNatural product
frameworksNatural products
HH
OH OAcH
HO O OH
OHO
OBz
P450
P450
P450
P450
P450 P450 Oxidativeauxiliaryenzymes
Taxadiene
OP2O63ndash
OP2O63ndash
3times
(a)
(b)
10-Deacetylbaccatin III
DMAPP
IPPand electrophilic
cyclizations
SCHEME 13 (a) From building blocks to natural products and (b) the example of 10‐deacetylbaccatin III
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 5
farnesylfarnesyl (C30
triterpene precursor) diphosphates can also be obtained by further additions of IPP leading to longer linear intermediates
The cyclization of linear precursors is achieved by speshycialized cyclases which generate a poorly functionalized natural product framework [10 11] Auxiliary enzymes such as oxygenases then increase the complexity and the diversity
of compounds by further functionalization (Scheme 13b) [12] A high degree of oxidation can be observed in comshypounds like thapsigargin paclitaxel or bilobalide (Fig 13) The biosynthesis of this last compound for example involves a high oxygenation pattern two Wagnerndashmeerwein rearrangements and several oxidative cleavages leading to the loss of five carbons The resulting natural products can
KS
S-ACP
O O
KR
S-ACP
OH O
S-MAT
O
YesNo
R
R
R
YesNoDH
S-ACP
OR
YesNoER
S-ACP
OR
YesNoTE
OH
OR
New cycle
New cycleor
release
New cycleor
release
New cycleor
release
Release
R in
crem
ente
d by
two
carb
ons
HO
O
S-ACP
O
Claisen condensation (ndashCO2)
reduction (NADPH)
dehydration (ndashH2O)
+
reduction (NADPH)
hydrolysis (H2O)
O O NH
OH
OH
H H
H H
Hirsutellone B(mixed PKSNRPS
product)
OO
O
OHOH
HO OH
Norsolorinic acid
O
O
O
O CHO
OH
HH
H
H
HH
OHHemibrevetoxin B
OOHO
OHO
HOHO OH
Erythronolide A
OH
OH
HO
Panaxytriol
From ahexanoylstartingblock
H
H
Fromtyrosine
H
1C lost fromdecarboxylation
FIGURE 12 Chemical logic of polyketide construction leading to variable functionalization of the elongated acyl chain and examples of resulting chemical diversity ACP acyl carrier protein DH dehydratase ER enoyl reductase KR ketoreductase KS ketosynthase mAT malonyl acyl transferase TE thioesterase
DMAPPOP2O6
3ndashOP2O6
3ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
Geranyl diphosphate (GPP)2 times IPP
H H
Geranylgeranyl diphosphate (GGPP)
Examplesverticillane
casbanetaxane
phorbol
Exampleslabdane
pimaranekauraneabietane
aphidicolanegibberellane
IPP
SCHEME 14 Early assembly of C5 units in terpene biosynthesis leading to diterpenes (C
20)
6 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
thus be extremely modified with structures whose biogenetic origin is far from being obvious at first sight and cannot be determined without further experiments such as isotopic labeling
1153 Flavonoids Resveratrols Gallic Acids and Further Polyphenolics We have previously discussed the polyketide origin of some phenolic compounds based on the (C
2)
n motif Other polyphenols like gallic acids are directly
derived by the aromatization of shikimic acid (C6C
1 building
block Scheme 12) [13] The C6C
3 building blocks are availshy
able from the conversion of phenylalanine and tyrosine into cinnamic and p‐coumaric acids respectively and then by further hydroxylation steps (Scheme 15) These can dimerize into lignans (eg podophyllotoxin) [14 15] through radical processes or converted to low molecular weight compounds like eugenol coumarins or vanillin [16] The coenzyme A thioesters of these C
6C
3 acids can be used as initiator units
by specialized ketosynthases for an elongation by two acetyl units leading to aromatic polyketides like styrylpyrones or diarylheptanoids (eg curcumin) [17] Important comshypounds from this metabolism are flavonoids (C
6C
3C
6) [18]
and stilbenoids (C6C
2C
6) (a decarboxylation occurs during
the aryl cyclization) [19] which are synthesized by chalcone synthase and stilbene synthase respectively Flavonoids (eg catechin) and stilbenes (eg resveratrol) are present in large amounts in fruits and vegetables and may exert their radical scavenging properties in vivo
1154 Alkaloids Alkaloids are nitrogen‐containing compounds The nitrogen(s) can be involved in an amine function conferring basicity to the natural product (like ldquoalkalirdquo) or in less or nonbasic functions such as an amide a nitrile an isonitrile or an ammonium salt (quaternary amines) For amines protonation often occurs at physiological pH and may condition their biological activity In many cases the nitrogen is biogenetically derived from an amino acid We will thus discuss alkaloids according to their amino acid origin
Alkaloids Derived from the Krebs Cycle (Lysine and Ornithine Derived) As shown previously (Scheme 12) the Krebs cycle is a crucial metabolic process which leads to α‐ketoacids (oxaloacetic and 2‐oxoglutaric acids) Their enzymatic transamination affords the two amino acidsmdashaspartic acid and glutamic acidmdashwhich are the direct biosynthetic precursors of amino acids lysine and ornithine respectively These in turn produce cadaverine a ldquoC
5Nrdquo unit
and putrescine a ldquoC4Nrdquo unit which are major components
for the biosynthesis of important alkaloids as will be discussed later (Scheme 16) Additionally ornithine is a precursor of arginine another important amino acid
ornithine‐derived alkaloids (incorporating the c4n
unit) Putrescine is derived from the decarboxylation of ornithine and is a precursor of linear polyamines like spermshyine After enzymatic methylation of one amine of putrescine
C10 C15
C15
C10
C10
C30
CO2H
Chrysanthemic acid
OH
Menthol
O
HO
H
HOGlc
MeO2CLoganin (iridoid)
H
H
H
HH
OO
OParthenolide
O
O
OHOHO
OAcHO
OnC3H7
O
O
O
nC7H15
Thapsigargin
O
O
OO
H
H
O
Cleavage
H
H
Artemisinin
C20 C40
O
O
OO
OO
OHOHH
Bilobalide(highly rearrangedand missing 5 C)
CleavageH
Highoxidativecleavages
HO OAcH
AcO O OH
OOOBz
O
Ph
BzNH
OHFrom PHE Paclitaxel
C25O
H
HO
OHCH
OH Ophiobolin A
H
HO
H
H
HHCholesterol
(missing 3 C WM)
H
H H H
H
H
Hopene
β-Carotene
C30 - 3
C15
C20 - 5WM
WM
FIGURE 13 Chemical diversity in the terpene series (Wm Wagnerndashmeerwein shifts lost carbons bold bonds are remnant of primary building blocks)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 7
in the presence of S‐adenosylmethionine transamination of the other affords γ‐(N‐methylamino)aldehyde [20] The resulting cyclic iminium is a key intermediate in the formation of many medicinally important alkaloids such as
the plant‐derived compounds cocaine atropine or the calysshytegines [21 22] Indeed this iminium is a mannich acceptor which can react with various nucleophiles the first of those being the carbanion of acetyl‐CoA Thus after a stepwise
CO2H
RCinnamic acid (R=H)p-Coumaric acid (R=OH)
RO
PHETYR
OH
[H] [O]
lignans
OH
O
O
O
O
OMeMeO
OMe
C mdash C andor C mdash Oradical couplings
After E Zisomerization
for example Podophyllotoxin
O ORO
Coumarins(eg scopoletin)
[O]
Prenylationcyclizationcleavage[O]
O OO
Furocoumarins(eg psoralen)
RO
nAcetyl-CoA
thencyclization
n = 2 styrylpyrones
O O
OMe
n = 3 avonoids(from chalcone synthase)
C6C3
H
H H
From AcCoA
O
n = 3 stilbenoids(from stilbene synthase)
HO
OH
OHFrom AcCoA From AcCoA(ndashCO2)
Resveratrol
HO
OH
OH
OH
OH
H
Catechin
MeO Yangonin
From AcCoA
SCHEME 15 The phenylpropanoid biosynthetic pathways
LYS
CO2
Cadaverine
O NH
H2O
NH
Piperideineiminium
Pelletierine
AcAcCoA
CO2
NH
O
N
O
Pseudopelletierine
NH
Ph
OH
Ph
O
Lobeline
NH
δ+
δndash
N
H
H
N
H
OH
Lupinine
N
H N
H
(+)-Sparteine
N
NH
O (ndash)-Cytisine
NH
CO2H
Pipecolic acid
N
OHHO
OH
HO
H
Castanos permine
ORN
CO2
NH2 NH2 NH2 NH2
NH2
NH2
NH2
PutrescineO
NHMe
N-Methylpyrrolinium
2 timesAcCoA
NMe
ARG
n = 1 spermidinen = 2 homospermidine
NMe
O
HygrineCO2
CO2
MeN
O
[O]
TropinoneCO2
HNHO
OHOH
OH
Calystegine B2
MeN
OAtropine
O
Ph
HO
NMe
NMe
NMe
O
Cuscohygrine
HN
HN
NH2
( )n
O
n = 2
N
HHO
Retronecine
HH
H
H
H
HO
H H
H
SCHEME 16 Lysine‐ and ornithine‐derived alkaloid biosynthetic pathways (mind the structural similarities)
8 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
elongation by two AcCoA units either decarboxylation can occur leading to the acetonylpyrrolidine hygrine or a secshyond mannich reaction by the intramolecular attack of the acetoacetate anion onto an oxidation‐derived pyrrolinium leading to the tropane skeleton (tropinone) The acetoacetyl‐CoA intermediate can also react intermolecularly with another pyrrolinium cation leading to cuscohygrine after decarboxylation Finally the pyrrolizidine alkaloids [23] are derived from homospermidine which when submitted to terminal oxidative deamination leads to the bicyclic skelshyeton of retronecine and further Senecio alkaloids We can mention herein that ornithine is a biosynthetic precursor of arginine bearing a guanidine function which is an intermediate toward the toxic compounds tetrodotoxin and saxitoxin (not shown)
lysine‐derived alkaloids (incorporating the c5n
unit) From lysine to piperidine alkaloids the biosynthetic steps parallel the one previously described from ornithine Indeed the oxidative deamination of cadaverine affords a δ‐amino aldehyde which cyclizes through imine formation into piperideine Protonation results in a mannich acceptor which is able to react with various nucleophiles such as β‐ketothioester anions The first product of these reactions is pelletierine which can further react through an intramolecular mannich
reaction leading to pseudopelletierine Quinolizidines [24] can also be formed first from the mannich reaction of the piperideine acceptor with the corresponding enamine nucleoshyphile and then after additional transformation steps leading for example to lupinine sparteine or cytisine
Indolizidine alkaloids [15] such as castanospermine and swainsonine are formed from pipecolic acid an amino acid derived from lysine which can be elongated by malonyl‐CoA followed by ring closure When protonated these alkaloids are oxonium mimics strongly inhibiting glycosidases
Tyrosine‐ and Phenylalanine‐Derived Alkaloids Tyrosine and phenylalanine amino acids are bearing the phenylshyethylamine moiety of many medicinally relevant alkaloids Further hydroxylations on the aromatic carbocycle or on the aliphatic part can be observed methylations can occur on phenolic oxygens and on the amine leading to catecholamines (adrenaline noradrenaline dopamine) Arylethylamines are also usual to react with endogenous aldehydes through PictetndashSpengler reactions [25] leading to important biosynthetic intermediates (Scheme 17) like
bull Reticuline from the reaction with 4‐hydroxyphenylacetshyaldehyde toward benzyltetrahydroisoquinoline alkaloids
NH2
Phenylethylamines
RONH TetrahydroisoquinolinesRO
RʹCHO
Rʹ
NH
(CH2)n
MeO
HO
Benzyltetrahydroisoquinoline(n = 1) for example reticuline
orPhenethyltetrahydroisoquinoline
(n = 2) eg autumnaline
IntermolecularCmdashO phenol
couplings
Curarealkaloids
IntramolecularC mdash C phenol
couplings
ONMe
HHO
H
HO
Morphine
NMe
MeO
HO
MeOOH
Isoboldine
O
O
OMe
NO2
CO2H
Aristolochic acid
OMe
O
NHAcMeO
MeOMeO
Colchicine
N
O
O
OMe
OMeBerberine
IntramolecularCmdash C coupling
and furtheroxidative events
Rʹ derivedfrom PHE
Pictet-Spenglerreaction
TYR
HO
HO CHO
HNHO
HO
OH
OMeO
OH
NMe
[O]
GalantamineNorbelladine
Rʹ derived fromsecologanin
NAc
HO
HO
O
CO2MeH
H
H
OHIpecosideaglycone
Ipecac alkaloids
1ClostCmdash C cleavage
and ring extension
H
ROH
1C lost
Cleavage
SCHEME 17 Tyrosine‐derived alkaloid biosynthetic pathways (double head arrows figure bond cleavages during biosynthetic processes)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 9
morphine berberine tubocurarine isoboldine or the highly modified aristolochic acid [26 27]
bull Automnaline from the reaction with 3‐(4‐hydroxyphenyl)propanal toward phenylethyltetrahydroisoquinoline alkaloids colchicine cephalotaxine or schelhammerishycine [28 29]
bull Ipecoside from the reaction of dopamine with secoloshyganin toward terpene tetrahydroisoquinoline alkaloids ipecoside or emetine
Lastly norbelladine (top of Scheme 17) is issued from the reductive amination of 34‐dihydroxybenzaldehyde (derived from phenylalanine) with tyramine (derived from tyrosine) and constitutes a biosynthetic node leading to Amaryllidaceae alkaloids such as galantamine crinine or lycorine depending on the topology of phenolic couplings In all these biosynthetic routes radical phenolic couplings are key reactions for CC and CO bond formations and rearrangements [30 31]
Tryptophan‐Derived Indole and Indole Monoterpene Alkaloids As for alkaloids derived from tyrosine and phenylalanine those derived from tryptophan are formed after decarboxylation of the amino acid (into tryptamine) and possible hydroxylation of the aromatic carbocycle (eg serotonin) and N‐methylation (eg psilocin) As previously tryptamine can also react through PictetndashSpengler reactions to form tetrahydro‐β‐carbolines which can be aromatized for example into harmine (Scheme 18) [16]
When the aldehyde partner of the PictetndashSpengler reacshytion with tryptamine is the terpene secologanin strictosidine is formed as an entry toward the vast monoterpene indole alkaloids [32 33] Hydrolysis of the glucosidic part releases the strictosidine aglycone bearing an aldehyde while iminshyium formation and further cyclization and reduction can lead to ajmalicine (from oxocyclization) or yohimbine (from carshybocyclization) These alkaloids are referred to as from the Corynanthe type with the monoterpene carbon skeleton unmodified Although it misses one carbon and has a very
RO
Carbonylcompounds
TRPNH
NH2
Indolethylamines(eg serotonine)
RONH
NH
Rʹ NH
N
MeOPictet-Spenglerreaction for example Harmineβ-Carbolines
NH
NHH
O
MeO2C
MeO2C
OH
H
H
Rʹ derived from secologanin
Strictosidine aglycone
NH
N
OH
H
H
Ajmalicine
N
N
O
O
H
H
H
Strychnine (C lost)Corynanthe type
Aspidosperma type Iboga type
N
N
OH
OMe
Quinine (C lost)
N
NH CO2Me
Catharanthine
NH
N
CO2MeH
OAc
OH
Vindoline
Complextransformations
NH
NMe
HN
MeN
H
H
Chimonanthine
Cyclizationdimerization
Prenylationcyclization
NH
NMeHO2C
H
Lysergic acid
H
H
Ergot alkaloidsPyrroloindoles Indole monoterpene
1C lost
1C lostFrom AcCoA
SCHEME 18 Tryptophan‐derived alkaloid biosynthetic pathways (gray parts monoterpenic units)
10 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
different structure strychnine is related to the Corynanthe alkaloids incorporating two carbons from acetyl‐CoA Highly modified monoterpene skeletons are derived from the Corynanthe core through CC bond breaking and reorshyganization leading to Iboga‐type (eg catharanthine) and Aspidosperma‐type (eg vindoline) alkaloids The antishycancer drug vinblastine is a heterodimer resulting from the nucleophilic attack of vindoline on a mannich acceptor resulting from catharanthine found in madagascar perishywinkle (Catharanthus roseus) The heteroaromatic comshypounds ellipticine camptothecin and quinine are also derived from a Corynanthe‐type precursor although in this case the biosynthetic relationship may not be obvious due to deep modifications of the skeleton
Finally two important classes of compounds have to be mentioned since they have inspired many synthetic chemists The pyrroloindole alkaloids result from the cyclization of tryptamine as found in physostigmine (formed by a cationic mechanism after methylation in position 3 of the indole not shown) or in chimonanthine (presumably formed by a radical coupling mechanism Scheme 18) The ergot alkaloids are derived from the 33‐dimethylallylation on position 4 of the indole in trypshytophan whose further cyclization and oxidation processes afford the natural products (eg lysergic acid Scheme 18 and ergotamine) which have had important medical applications [34]
NRPS Metabolites and Peptides NRPS enzymes assemble amino acids including nonproteinogenic ones into oligopeptides The enzymes contain several modules and especially an adenylation domain (A) which specifically selects and activates the amino acid to be transferred as a thioester on the nearby peptidyl carrier protein (PCP) [2] A condensation module (C) then catalyzes the formation of the peptide bonds between the newly introduced amino acyl‐PCP (bearing a free amine) and the elongated peptidyl‐PCP thioester At the end of the elongation a cyclization can occur into cyclopeptides but the peptide can also be
transferred to auxiliary enzymes like methyltransferases glycosyltransferases or oxidases (vancomycins are typical products of such functionalizations) [35 36] The formation of heterocycles is also frequently encountered in this metabolism as in penicillins that are derived from the tripeptide α‐aminoadipoyl‐cysteinyl‐valine or telomestatin (Fig 14) [2]
Other Alkaloid Origins There are many other nitrogen sources involved in alkaloid biosyntheses for example nicotinic acid (originated from aspartic acid and intershymediate in nicotine and anabasine biosyntheses) and anthranilic acid (originated from tryptophan and intermedishyate toward acridines or aurachins) The amination reaction (eg through transamination of carbonyl compounds) is also a way to introduce nitrogens in natural products for example from fatty acids steroids (toward Solanum alkaloids or cyclopamines) or other terpenoids (aconitine and atisine have diterpene skeletons while Daphniphyllum alkaloids are triterpene derivatives) Finally nucleic acids can also be precursors of alkaloids like the well‐known caffeine
12 FROM BIOSYNTHESIS TO TOTAL SYNTHESIS STRATEGIES TOWARD THE NATURAL PRODUCT CHEMICAL SPACE
121 The Chemical Space of Natural Products
Natural products occupy an important place in human com munities as demonstrated by their vast use from ancient times to nowadays like dyes fibers oils pershyfumes agrochemicals or drugs Broadly both primary and secondary metabolites could be classified as natural products while the latter as discussed previously are usushyally regarded as the ldquonatural productsrdquo owing to their comshyplexity and diversity arising from a variety of biosynthetic pathways The structural chemical diversity found among
N
S
CO2HO
HNPhH2C
O
Penicillin G
OH
HN
HN
O
O
ONH2
O
OHO
NHMe
OCl
O
NH
NH
NH
O
ClHO
O
HN
H
H
HOOH
OHHO2C
Vancomycin aglycone
ON
NO
O
NN
O
O
N
N O
Me
S
N N
OMeH
Telomestatin
L-AAA-L-CYS-D-VAL
Enzymes
FIGURE 14 Structural diversity of nonribosomal peptide compounds (AAA α‐aminoadipic acid)
FROm BIOSyNTHESIS TO TOTAL SyNTHESIS STRATEGIES TOWARD THE NATuRAL PRODuCT CHEmICAL SPACE 11
all living organisms defining the chemical space of natural products [37] is the consequence of their evolution occurshyring as an adaptation of organisms to their environment It is commonly believed that secondary metabolites are proshyduced as messengers by living organisms or as weapons against enemies and thus they should have certain biological activities in a medicinal point of view [38] Indeed natural products are regarded as one of the main sources of medicines (Fig 15) From the traditional medicinal extracts to every single bioactive molecule the methods of extraction purification identification and biological investigation of natural products have been well established Their complex structures and interesting properties have attracted synthetic chemists to accomplish their total syntheses and that of medicinally relevant anashylogs sometimes in the industrial context [39] Thus tarshygeting the chemical space of natural products has never been more relevant than today Although the discovery of natural products demands time and labor‐consuming manipulations it is worth to notice that the knowledge on this chemical space is still continually growing while biological advances allow for discovering and undershystanding potential targets However increasing the chemical space of human‐made compounds based on natural products should benefit from transdisciplinary colshylaborations such as the use of coupled biosynthetic and chemical synthetic methods to design original ldquounnatural natural productsrdquo [40]
122 The Biosynthetic Pathways as an Inspiration for Synthetic Challenges
1221 Precursor‐Directed Biosyntheses and Mutashysynthetic Strategies to Increase the Chemical Space of Natural Products As the genetics and biochemistry of natural product biosynthesis are better understood novel biosynthetic techniques have been developed to study and generate new diversity in natural product analogs (Scheme 19) Precursor‐directed biosynthesis (PDB) is considered as the earliest example of combining chemical and biological methods for the generation of complex natural product analogs [41 42] This approach compared with the biosynthetic pathway of wild‐type metabolites (Scheme 19a) involves the feeding of analogs of the natural biosynthetic building blocks to the living organisms (Scheme 19b) usually bacteria or fungi which incorporate the modified precursors into the biosynthesized compound mutasynthesis also termed as mutational biosynthesis (mBS) involves the inactivation of a key step of the bioshysynthesis in a mutant microorganism (Scheme 19c) which can then be fed by various modified or advanced building blocks (mutasynthons Scheme 19d) [43] These mutasshyynthons could not be incorporated by the wild type due to specificities of the enzymatic machinery Build up on PDB mBS eliminates the natural biosynthetic intermediate thus generating a less complex mixture of metabolites and making the purification or yield of target products better
O NH
OHO
O
OO
O
O
OH
HOO
O O
OH
HOO
NH ONH2
O
ONH
NH
O
OCl Cl
O
O
OH
HN
HN
HN
HN
OO
HO
HN
OH
OHOH
O
O
HO
HO
OHO
O
OHH2N
O
OH3C
H
O
H
CH3
CH3
H
O O
NO
H
O
HO
O
NO
O
NH
HN
OHO
HONH
ON
H
HO
O
H2N
O OH
ONH
OH
ON OHO
OH
HO OS OH
OOOH
OHO
O
ON
OO
O
HO
O
O OH
OO
Micafunginantifungal
Rapamycinimmunosuppressive
Galantamineanti-Alzheimer
Taxolanticancer
Artemisininantimalarial
Vancomycinantibiotic
FIGURE 15 Some famous natural products currently used as drugs
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product
There is pleasure in the pathless woodsthere is rapture in the lonely shore
there is society where none intrudesby the deep sea and music in its roar
I love not Man the less but Nature moreLord Byron
Preface
The first time I came across with the idea of editing a book that merges selected chapters of biosynthesis and total synthesis was when I was teaching postgraduate courses of natural product synthesis at Aristotle University of Thessaloniki This period I realized that the best way to teach youngsters synthesis was to start from the very origin of inspiration nature and its tools biosynthesis
Over the last decades biosynthesis is filling our gaps of understanding the complex mechanisms of nature and pro-vides useful sources of inspiration not only in the way natural products can be synthesized but also by directing synthetic chemists in developing atom‐economical efficient synthetic methods Several are the examples that mimic biosynthetic guidelines from modern iterative alkylations and aldol reactions to CH oxidations that compile nowadays the modern toolbox of organic synthesis
The handed book is constructed in the logic of presenting the parallel development of biosynthesis and organic meth-odology and how these can be applied in efficient syntheses of natural products The book is divided into four sections each representing the four major biosynthetic pathways of natural products namely acetate mevalonate shikimate biosynthetic pathways and the mixed biosynthetic pathways
of alkaloids These sections are divided into chapters that represent selected classes of natural products for example lipids sesquiterpenoids lignans etc Each of these chapters is further divided into three distinct subchapters (a) biosyn-thesis (b) methodological section and (c) application of the described methodology in the total synthesis of the described family of natural products By this way the readers can be focused in the direct comparison between biosyn-theses and the developed methodologies to construct the crucial for each class of natural product carbon bonds Although the book as it develops is focused on presenting the power of biosynthesis and how this power can be applied in providing inspiration for the efficient synthesis of natural products it was not the authors will to present only biomi-metic total syntheses but rather to exploit the modern synthetic methodologies and recognize their disabilities for further improvement
Of course this book will not have been realized without the excellent work of renowned scientists worldwide working either in the field of biosynthesis or total synthesis who collected the existing knowledge on biosynthesis analyzed the existing modern methodologies and presented a bouquet of selected total syntheses Throughout our
xvi PrEfACE
endeavor to complete this book I learned many things from their expertise but I also realized that only with tight collab-orations you can build long‐lasting friendships I would like to thank them all once again for their trust and effort to complete this book We all hope that the current work will contribute to a better understanding of the current status of
organic chemistry and to the discovery of novel strategies and tactics for the synthesis of natural products
Alexandros L ZografosSeptember 2015
Thessaloniki Greece
From Biosynthesis to Total Synthesis Strategies and Tactics for Natural Products First Edition Edited by Alexandros L Zografoscopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
11 FROM PRIMARY TO SECONDARY METABOLISM THE KEY BUILDING BLOCKS
111 Definitions
The primary and secondary metabolisms are traditionally distinguished by their distribution and utility in the living organism network Primary metabolites include carbohyshydrates lipids nucleic acids and proteins (or their amino acid constituents) and are shared by all living organisms on Earth They are transformed by common pathways which are studied by biochemistry (Fig 11) Secondary metabo-lites are structurally diverse compounds usually produced by a limited number of organisms which synthesize them for a special purpose like defense or signaling through specific biosynthetic pathways They are studied by natural product chemistry This distinction is not always so obvious and some compounds can be studied in the context of both primary and secondary metabolisms This is especially true nowadays with the use of genetic and biomolecular tools which tend to make natural product sciences more and more integrative However an important point to remember is that the primary metabolism furnishes key building blocks to the secondary metabolism It would be difficult to describe in detail the full biosynthetic pathshyways in this section We tried to organize the discussion as a vade mecum synthetically gathering information from extremely useful sources which will be cited at the end of this chapter
112 Energy Supply and Carbon Storing at the Early Stage of Metabolisms
The sunlight is essential to life except in some part of the deep oceans It provides energy for plant photosynthesis that splits molecules of water into protons and electrons and releases O
2 (Scheme 11) A proton gradient inside the plant
chloroplasts then drags a transmembrane ATP synthase comshyplex that produces adenosine triphosphate (ATP) while elecshytrons released from water are transferred to the coenzyme reducer nicotinamide adenine dinucleotide phosphate hydride (NADPH) A major function of chloroplasts is to fix CO
2 as a combination to ribulose‐15‐bisphosphate (RuBP)
performed by RuBP carboxylase (rubisco) forming an instable ldquoC
6rdquo β‐ketoacid This is cleaved into two molecules
of 3‐phosphoglycerate (3‐PGA) which is then reduced into 3‐phosphoglyceraldehyde (3‐PGAL a ldquoC
3rdquo triose phosshy
phate) during the Calvin cycle This is one of the major metabolites in the biosynthesis of carbohydrates like glucose and a biochemical mean for storing and retaining carbon atoms in the living cells
113 Glucose as a Starting Material Toward Key Building Blocks of the Secondary Metabolism
Glucose‐6‐phosphate arises from the phosphorylation of glucose It is the starting material of glycolysis an important process of the primary metabolism which consists in eight enzymatic reactions leading to pyruvic acid (PA)
FROM BIOSYNTHESES TO TOTAL SYNTHESES AN INTRODUCTION
Bastien Nay1 and Xu‐Wen Li2
1
1 Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France2 Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
2 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
(Scheme 12) Important intermediates for the secondary metabolism are produced during glycolysis Glucose glucose‐6‐phosphate and fructose‐6‐phosphate can be converted to other hexoses and pentoses that can be oligoshymerized and enter in the composition of heterosides Additionally fructose‐6‐phosphate connects the pentose
phosphate pathway leading to erythrose‐4‐phosphate toward shikimic acid which is a key metabolite in the biosynthesis of aromatic amino acids (phenylalanine tyrosine or C
6C
3
units) and C6C
1 phenolic compounds The next important
intermediate in glycolysis is 3‐PGAL which can be redishyrected toward methylerythritol‐4‐phosphate (mEP) in the
Primary metabolism Secondary metabolism
The field ofbiochemistry
The field ofnatural product
chemistry
Essential toliving organisms
Essential to theproducer organisms
under particularconditions
Nucleic acids (DNARNA) carbohydrates
lipids amino acidspeptides and proteins
Alkaloids terpenespolyketides
polyphenols andtheir heterosidic form
Biological effects(defense signaling)
Biosyntheticpathways
Main compoundclasses
FIGURE 11 Primary versus secondary metabolisms
PS-I
PS-IIendash
hν H2O H+ and O2
ATP synthase
NADP reductase
H+ (gradient)
H+ADP + Pi ATP
NADPHH+NADP+
H+
Thylakoidcompartment
Chloroplaststroma
Thylakoidmembrane
(a) Light dependent process
(b) Light independent process
CO2
RuBP
Rubisco
Unstable β-ketoacid3-PGA 13-diPGA
ATP
ADP3-PGAL
NADPHH+
NADP+
(Calvin cycle)
trioses tetroses pentoses hexoses (eg glucose) heptoses
Chloroplaststroma
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
CH2OPO32ndash CH2OPO3
2ndash CH2OPO32ndash
CH2OPO32ndash
OOH
HO
HOO
HO
HO2CCO2H
HO
CO2PO32ndash
HO
CHOHO
endash
Cytosol
SCHEME 11 The photosynthetic machinery (PS‐I and PS‐II photosystems I and II)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 3
chloroplast mEP is a starting block in the biosynthesis of terpenes through C
5 isoprene units (isopentenyl diphosphate
(IPP) and dimethylallyl diphosphate (DmAPP)) especially those in C
10 C
20 and C
40 terpenes 3‐PGA is a precursor of
serine and other amino acids while phosphoenolpyruvate (PEP) the precursor of PA is also an intermediate toward the previously mentioned shikimic acid Lastly PA is not only a precursor of the fundamental ldquoC
2rdquo acetyl coenzyme A
(AcCoA) unit but also an intermediate toward aliphatic amino acids and mEP
AcCoA is the building block of fatty acids polyketides and mevalonic acid (mVA) a cytosolic precursor of the C
5 isoprene units for the biosynthesis of terpenes in the
C15
and C30
series (mind it is different from the mEP pathway in product and in cell location) Finally AcCoA enters the citric acid or Krebs cycle which leads to several precursors of amino acids These are oxaloacetic acid precursor of aspartic acid through transamination (thus toward lysine as a nitrogenated C
5N linear unit and methishy
onine as a methyl supplier) and 2‐oxoglutaric acid preshycursor of glutamic acid (and subsequent derivatives such as ornithine as a nitrogenated C
4N linear unit) All these
amino acids are key precursors in the biosynthesis of many alkaloids
114 Reactions Involved in the Construction of Secondary Metabolites
most reactions occurring in the living cells are performed by specialized enzymes which have been classified in an intershynational nomenclature defined by an enzyme commission (EC) number There are six classes of enzymes depending on the biochemical reaction they catalyze EC‐1 oxidoreducshytases (catalyzing oxidoreduction reactions) EC‐2 transfershyases (catalyzing the transfer of functional groups) EC‐3 hydrolases (catalyzing hydrolysis) EC‐4 lyases (breaking bonds through another process than hydrolysis or oxidation leading to a new double bond or a new cycle) EC‐5 isomershyases (catalyzing the isomerization of a molecule) and EC‐6 ligases (forming a covalent bond between two molecules) many subclasses of these enzymes have been described depending on the type of atoms and functional groups involved in the reaction and if any on the cofactor used in this reaction For example several cofactors can be used by dehydrogenases like NAD(P)NAD(P)H FADFADH
2 or
FmNFmNH2 For a description of this classification the
reader can refer to specialized Internet websites like ExplorEnz [1] What is important to realize is that most enzymes are substrate specific and have been selected during
CHOHO
CO2H
O
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
HOOH
HO
HOH2CO
OHC
CH2OPO32ndash
OHHO
CO2H
HO
OH
OH
TYR
PHE
CO2H
NH2
TRP
CH2OPO32ndash
HO
HOH2COH
MEP
C10 C20 and C40 terpenes
CO2HHO
SER
GLY CYS
CO2HOPO3
2ndash
CH2OH
OHHO2C
MVA
C15 and C30 terpenes
Polyketides
Krebscycle
(citric acid)
VAL ALA ILE LEU
CO2H
O
HO2C
CO2HO
CO2H
ASP
LYS
GLU
Glucose-6-phosphate
Fructose-6-phosphate
Erythrose-4-phosphate
3-PGAL
OP2O63ndash OP2O6
3ndash
IPP DMAPP
3-PGA PEP PA
O SCoAAcCoA
3x
Cytosol
Cytosol
Chloroplasts
Chloroplasts
Glycolysis
C2
C5
Shikimic acid Oxaloaceticacid
2-Oxoglutaricacid
MET
THR
PRO
ARGORN
Alkaloids Alkaloids Alkaloids Alkaloids
Aminoacids
Peptidesproteins
Phenolics
C1
C5N C4N
ldquoCH3rdquo
(Indole)C2NC6C3C6C2N C6C1
Pentosephosphatepathway
SCHEME 12 The building block chart involving glycolysis and the Krebs cycle
4 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
evolution to perform specific transformations making natural products with often and yet unknown functions
Secondary metabolites arise from specific biosynthetic pathways which use the previously defined building blocks The bunch of organic reactions involved in these biosynshytheses allows the construction of natural product frameshyworks which are finally diversified through ldquodecorationrdquo steps (Scheme 13) It is not the purpose of this introductive chapter to describe in detail all biosynthetic pathways and the reader can refer to excellent books and articles which have been published elsewhere [2 3]
The reactions involved in the construction of natural product skeletons will be described later for representative classes of compounds The identification of the building block footprint in the natural product skeleton will be emphasized as much as possible sometimes referring to biogenetic speculations [4] After the framework construction the decoration steps will involve as diverse reactions as aliphatic CH oxidations (eg involving a cytochrome P
450 oxygenase) occasionally triggering a
rearrangement heteroatom alkylations (eg methylation by S‐adenosylmethionine) or allylation (by DmAPP) esterifications heteroatom or C‐glycosylations (leading to heterosides) radical couplings (especially for phenols) alcohol oxidations or ketone reductions amineketone transaminations alkene dihydroxylations or epoxidations oxidative halogenations BaeyerndashVilliger oxidations and further oxygenation steps At the end of the biosynthesis such transformations may totally hide the primary building block origin of natural products
115 Secondary Metabolisms
1151 Polyketides Polyketides (or polyacetates) are issued from the oligomerization of C
2 acetate units performed
by polyketide synthases (PKS) and leading to (C2)
n linear
intermediates [5 6] If the (C2)
n intermediates arise from
successive Claisen reactions performed by ketosynthase
domains (KS in nonreducing PKS) a highly reactive poly‐ β‐ketoacyl intermediate H(CH
2CO)
nOH is formed
leading to phenolic and aromatic products through further intramolecular Claisen condensations Furthermore highly reducing PKSs are made of specialized enzymatic subunits working in line or iteratively to functionalize each C
2 linker
bond as CH(OH)CH2 (by ketoreductases (KR)) then as
HCCH (by dehydratases (DH)) and as CH2CH
2 (by enoyl
reductases (ER)) leading to a high degree of functionalization of the final product (Fig 12) By these iterative sequences highly reduced polyketides which can be either linear macrocyclized or polycyclized depending on the reactivity of the biosynthetic intermediates can be formed [7] With the same logic fatty acids are also biosynthesized by fatty acid synthases
moreover the PKS enzyme can be hybridized with nonshyribosomal peptide synthetase (NRPS) domains (see also ldquoNRPS metabolites and peptidesrdquo in the ldquoAlkaloidsrdquo secshytion) leading to the acylation of an amino acid by the (C
2)
n
acyl intermediate As previously the functionalization of the acyl chain depends on the PKS enzyme and the PKSNRPS products are also extremely diversified (eg hirsutellone B Fig 12) [8]
1152 Terpenes Terpenes are derived from the oligoshymerization of the C
5 isoprene units DmAPP and IPP Both
precursors are prompt to generate either an allylic cation (the diphosphate is a good leaving group) or a tertiary carbocation respectively which makes the IPP easy to react with DmAPP (Scheme 14) This reaction happens in the active site of a terpene synthase which activates the deparshyture of the diphosphate group from DmAPP thanks to Lewis acid activation (a metal like mg2+ mn2+ or Co2+ is present in the enzyme active site [9]) This leads to geranyl (C
10
monoterpene precursor) or farnesyl (C15
sesquiterpene precursor) diphosphate depending on the location of the enzyme (chloroplast for the mEP pathway or cytosol for the mVA pathway) Geranylgeranyl (C
20 diterpene) and
Constructionreactions
Decoration reaction(functionalization)Building
blocksNatural product
frameworksNatural products
HH
OH OAcH
HO O OH
OHO
OBz
P450
P450
P450
P450
P450 P450 Oxidativeauxiliaryenzymes
Taxadiene
OP2O63ndash
OP2O63ndash
3times
(a)
(b)
10-Deacetylbaccatin III
DMAPP
IPPand electrophilic
cyclizations
SCHEME 13 (a) From building blocks to natural products and (b) the example of 10‐deacetylbaccatin III
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 5
farnesylfarnesyl (C30
triterpene precursor) diphosphates can also be obtained by further additions of IPP leading to longer linear intermediates
The cyclization of linear precursors is achieved by speshycialized cyclases which generate a poorly functionalized natural product framework [10 11] Auxiliary enzymes such as oxygenases then increase the complexity and the diversity
of compounds by further functionalization (Scheme 13b) [12] A high degree of oxidation can be observed in comshypounds like thapsigargin paclitaxel or bilobalide (Fig 13) The biosynthesis of this last compound for example involves a high oxygenation pattern two Wagnerndashmeerwein rearrangements and several oxidative cleavages leading to the loss of five carbons The resulting natural products can
KS
S-ACP
O O
KR
S-ACP
OH O
S-MAT
O
YesNo
R
R
R
YesNoDH
S-ACP
OR
YesNoER
S-ACP
OR
YesNoTE
OH
OR
New cycle
New cycleor
release
New cycleor
release
New cycleor
release
Release
R in
crem
ente
d by
two
carb
ons
HO
O
S-ACP
O
Claisen condensation (ndashCO2)
reduction (NADPH)
dehydration (ndashH2O)
+
reduction (NADPH)
hydrolysis (H2O)
O O NH
OH
OH
H H
H H
Hirsutellone B(mixed PKSNRPS
product)
OO
O
OHOH
HO OH
Norsolorinic acid
O
O
O
O CHO
OH
HH
H
H
HH
OHHemibrevetoxin B
OOHO
OHO
HOHO OH
Erythronolide A
OH
OH
HO
Panaxytriol
From ahexanoylstartingblock
H
H
Fromtyrosine
H
1C lost fromdecarboxylation
FIGURE 12 Chemical logic of polyketide construction leading to variable functionalization of the elongated acyl chain and examples of resulting chemical diversity ACP acyl carrier protein DH dehydratase ER enoyl reductase KR ketoreductase KS ketosynthase mAT malonyl acyl transferase TE thioesterase
DMAPPOP2O6
3ndashOP2O6
3ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
Geranyl diphosphate (GPP)2 times IPP
H H
Geranylgeranyl diphosphate (GGPP)
Examplesverticillane
casbanetaxane
phorbol
Exampleslabdane
pimaranekauraneabietane
aphidicolanegibberellane
IPP
SCHEME 14 Early assembly of C5 units in terpene biosynthesis leading to diterpenes (C
20)
6 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
thus be extremely modified with structures whose biogenetic origin is far from being obvious at first sight and cannot be determined without further experiments such as isotopic labeling
1153 Flavonoids Resveratrols Gallic Acids and Further Polyphenolics We have previously discussed the polyketide origin of some phenolic compounds based on the (C
2)
n motif Other polyphenols like gallic acids are directly
derived by the aromatization of shikimic acid (C6C
1 building
block Scheme 12) [13] The C6C
3 building blocks are availshy
able from the conversion of phenylalanine and tyrosine into cinnamic and p‐coumaric acids respectively and then by further hydroxylation steps (Scheme 15) These can dimerize into lignans (eg podophyllotoxin) [14 15] through radical processes or converted to low molecular weight compounds like eugenol coumarins or vanillin [16] The coenzyme A thioesters of these C
6C
3 acids can be used as initiator units
by specialized ketosynthases for an elongation by two acetyl units leading to aromatic polyketides like styrylpyrones or diarylheptanoids (eg curcumin) [17] Important comshypounds from this metabolism are flavonoids (C
6C
3C
6) [18]
and stilbenoids (C6C
2C
6) (a decarboxylation occurs during
the aryl cyclization) [19] which are synthesized by chalcone synthase and stilbene synthase respectively Flavonoids (eg catechin) and stilbenes (eg resveratrol) are present in large amounts in fruits and vegetables and may exert their radical scavenging properties in vivo
1154 Alkaloids Alkaloids are nitrogen‐containing compounds The nitrogen(s) can be involved in an amine function conferring basicity to the natural product (like ldquoalkalirdquo) or in less or nonbasic functions such as an amide a nitrile an isonitrile or an ammonium salt (quaternary amines) For amines protonation often occurs at physiological pH and may condition their biological activity In many cases the nitrogen is biogenetically derived from an amino acid We will thus discuss alkaloids according to their amino acid origin
Alkaloids Derived from the Krebs Cycle (Lysine and Ornithine Derived) As shown previously (Scheme 12) the Krebs cycle is a crucial metabolic process which leads to α‐ketoacids (oxaloacetic and 2‐oxoglutaric acids) Their enzymatic transamination affords the two amino acidsmdashaspartic acid and glutamic acidmdashwhich are the direct biosynthetic precursors of amino acids lysine and ornithine respectively These in turn produce cadaverine a ldquoC
5Nrdquo unit
and putrescine a ldquoC4Nrdquo unit which are major components
for the biosynthesis of important alkaloids as will be discussed later (Scheme 16) Additionally ornithine is a precursor of arginine another important amino acid
ornithine‐derived alkaloids (incorporating the c4n
unit) Putrescine is derived from the decarboxylation of ornithine and is a precursor of linear polyamines like spermshyine After enzymatic methylation of one amine of putrescine
C10 C15
C15
C10
C10
C30
CO2H
Chrysanthemic acid
OH
Menthol
O
HO
H
HOGlc
MeO2CLoganin (iridoid)
H
H
H
HH
OO
OParthenolide
O
O
OHOHO
OAcHO
OnC3H7
O
O
O
nC7H15
Thapsigargin
O
O
OO
H
H
O
Cleavage
H
H
Artemisinin
C20 C40
O
O
OO
OO
OHOHH
Bilobalide(highly rearrangedand missing 5 C)
CleavageH
Highoxidativecleavages
HO OAcH
AcO O OH
OOOBz
O
Ph
BzNH
OHFrom PHE Paclitaxel
C25O
H
HO
OHCH
OH Ophiobolin A
H
HO
H
H
HHCholesterol
(missing 3 C WM)
H
H H H
H
H
Hopene
β-Carotene
C30 - 3
C15
C20 - 5WM
WM
FIGURE 13 Chemical diversity in the terpene series (Wm Wagnerndashmeerwein shifts lost carbons bold bonds are remnant of primary building blocks)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 7
in the presence of S‐adenosylmethionine transamination of the other affords γ‐(N‐methylamino)aldehyde [20] The resulting cyclic iminium is a key intermediate in the formation of many medicinally important alkaloids such as
the plant‐derived compounds cocaine atropine or the calysshytegines [21 22] Indeed this iminium is a mannich acceptor which can react with various nucleophiles the first of those being the carbanion of acetyl‐CoA Thus after a stepwise
CO2H
RCinnamic acid (R=H)p-Coumaric acid (R=OH)
RO
PHETYR
OH
[H] [O]
lignans
OH
O
O
O
O
OMeMeO
OMe
C mdash C andor C mdash Oradical couplings
After E Zisomerization
for example Podophyllotoxin
O ORO
Coumarins(eg scopoletin)
[O]
Prenylationcyclizationcleavage[O]
O OO
Furocoumarins(eg psoralen)
RO
nAcetyl-CoA
thencyclization
n = 2 styrylpyrones
O O
OMe
n = 3 avonoids(from chalcone synthase)
C6C3
H
H H
From AcCoA
O
n = 3 stilbenoids(from stilbene synthase)
HO
OH
OHFrom AcCoA From AcCoA(ndashCO2)
Resveratrol
HO
OH
OH
OH
OH
H
Catechin
MeO Yangonin
From AcCoA
SCHEME 15 The phenylpropanoid biosynthetic pathways
LYS
CO2
Cadaverine
O NH
H2O
NH
Piperideineiminium
Pelletierine
AcAcCoA
CO2
NH
O
N
O
Pseudopelletierine
NH
Ph
OH
Ph
O
Lobeline
NH
δ+
δndash
N
H
H
N
H
OH
Lupinine
N
H N
H
(+)-Sparteine
N
NH
O (ndash)-Cytisine
NH
CO2H
Pipecolic acid
N
OHHO
OH
HO
H
Castanos permine
ORN
CO2
NH2 NH2 NH2 NH2
NH2
NH2
NH2
PutrescineO
NHMe
N-Methylpyrrolinium
2 timesAcCoA
NMe
ARG
n = 1 spermidinen = 2 homospermidine
NMe
O
HygrineCO2
CO2
MeN
O
[O]
TropinoneCO2
HNHO
OHOH
OH
Calystegine B2
MeN
OAtropine
O
Ph
HO
NMe
NMe
NMe
O
Cuscohygrine
HN
HN
NH2
( )n
O
n = 2
N
HHO
Retronecine
HH
H
H
H
HO
H H
H
SCHEME 16 Lysine‐ and ornithine‐derived alkaloid biosynthetic pathways (mind the structural similarities)
8 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
elongation by two AcCoA units either decarboxylation can occur leading to the acetonylpyrrolidine hygrine or a secshyond mannich reaction by the intramolecular attack of the acetoacetate anion onto an oxidation‐derived pyrrolinium leading to the tropane skeleton (tropinone) The acetoacetyl‐CoA intermediate can also react intermolecularly with another pyrrolinium cation leading to cuscohygrine after decarboxylation Finally the pyrrolizidine alkaloids [23] are derived from homospermidine which when submitted to terminal oxidative deamination leads to the bicyclic skelshyeton of retronecine and further Senecio alkaloids We can mention herein that ornithine is a biosynthetic precursor of arginine bearing a guanidine function which is an intermediate toward the toxic compounds tetrodotoxin and saxitoxin (not shown)
lysine‐derived alkaloids (incorporating the c5n
unit) From lysine to piperidine alkaloids the biosynthetic steps parallel the one previously described from ornithine Indeed the oxidative deamination of cadaverine affords a δ‐amino aldehyde which cyclizes through imine formation into piperideine Protonation results in a mannich acceptor which is able to react with various nucleophiles such as β‐ketothioester anions The first product of these reactions is pelletierine which can further react through an intramolecular mannich
reaction leading to pseudopelletierine Quinolizidines [24] can also be formed first from the mannich reaction of the piperideine acceptor with the corresponding enamine nucleoshyphile and then after additional transformation steps leading for example to lupinine sparteine or cytisine
Indolizidine alkaloids [15] such as castanospermine and swainsonine are formed from pipecolic acid an amino acid derived from lysine which can be elongated by malonyl‐CoA followed by ring closure When protonated these alkaloids are oxonium mimics strongly inhibiting glycosidases
Tyrosine‐ and Phenylalanine‐Derived Alkaloids Tyrosine and phenylalanine amino acids are bearing the phenylshyethylamine moiety of many medicinally relevant alkaloids Further hydroxylations on the aromatic carbocycle or on the aliphatic part can be observed methylations can occur on phenolic oxygens and on the amine leading to catecholamines (adrenaline noradrenaline dopamine) Arylethylamines are also usual to react with endogenous aldehydes through PictetndashSpengler reactions [25] leading to important biosynthetic intermediates (Scheme 17) like
bull Reticuline from the reaction with 4‐hydroxyphenylacetshyaldehyde toward benzyltetrahydroisoquinoline alkaloids
NH2
Phenylethylamines
RONH TetrahydroisoquinolinesRO
RʹCHO
Rʹ
NH
(CH2)n
MeO
HO
Benzyltetrahydroisoquinoline(n = 1) for example reticuline
orPhenethyltetrahydroisoquinoline
(n = 2) eg autumnaline
IntermolecularCmdashO phenol
couplings
Curarealkaloids
IntramolecularC mdash C phenol
couplings
ONMe
HHO
H
HO
Morphine
NMe
MeO
HO
MeOOH
Isoboldine
O
O
OMe
NO2
CO2H
Aristolochic acid
OMe
O
NHAcMeO
MeOMeO
Colchicine
N
O
O
OMe
OMeBerberine
IntramolecularCmdash C coupling
and furtheroxidative events
Rʹ derivedfrom PHE
Pictet-Spenglerreaction
TYR
HO
HO CHO
HNHO
HO
OH
OMeO
OH
NMe
[O]
GalantamineNorbelladine
Rʹ derived fromsecologanin
NAc
HO
HO
O
CO2MeH
H
H
OHIpecosideaglycone
Ipecac alkaloids
1ClostCmdash C cleavage
and ring extension
H
ROH
1C lost
Cleavage
SCHEME 17 Tyrosine‐derived alkaloid biosynthetic pathways (double head arrows figure bond cleavages during biosynthetic processes)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 9
morphine berberine tubocurarine isoboldine or the highly modified aristolochic acid [26 27]
bull Automnaline from the reaction with 3‐(4‐hydroxyphenyl)propanal toward phenylethyltetrahydroisoquinoline alkaloids colchicine cephalotaxine or schelhammerishycine [28 29]
bull Ipecoside from the reaction of dopamine with secoloshyganin toward terpene tetrahydroisoquinoline alkaloids ipecoside or emetine
Lastly norbelladine (top of Scheme 17) is issued from the reductive amination of 34‐dihydroxybenzaldehyde (derived from phenylalanine) with tyramine (derived from tyrosine) and constitutes a biosynthetic node leading to Amaryllidaceae alkaloids such as galantamine crinine or lycorine depending on the topology of phenolic couplings In all these biosynthetic routes radical phenolic couplings are key reactions for CC and CO bond formations and rearrangements [30 31]
Tryptophan‐Derived Indole and Indole Monoterpene Alkaloids As for alkaloids derived from tyrosine and phenylalanine those derived from tryptophan are formed after decarboxylation of the amino acid (into tryptamine) and possible hydroxylation of the aromatic carbocycle (eg serotonin) and N‐methylation (eg psilocin) As previously tryptamine can also react through PictetndashSpengler reactions to form tetrahydro‐β‐carbolines which can be aromatized for example into harmine (Scheme 18) [16]
When the aldehyde partner of the PictetndashSpengler reacshytion with tryptamine is the terpene secologanin strictosidine is formed as an entry toward the vast monoterpene indole alkaloids [32 33] Hydrolysis of the glucosidic part releases the strictosidine aglycone bearing an aldehyde while iminshyium formation and further cyclization and reduction can lead to ajmalicine (from oxocyclization) or yohimbine (from carshybocyclization) These alkaloids are referred to as from the Corynanthe type with the monoterpene carbon skeleton unmodified Although it misses one carbon and has a very
RO
Carbonylcompounds
TRPNH
NH2
Indolethylamines(eg serotonine)
RONH
NH
Rʹ NH
N
MeOPictet-Spenglerreaction for example Harmineβ-Carbolines
NH
NHH
O
MeO2C
MeO2C
OH
H
H
Rʹ derived from secologanin
Strictosidine aglycone
NH
N
OH
H
H
Ajmalicine
N
N
O
O
H
H
H
Strychnine (C lost)Corynanthe type
Aspidosperma type Iboga type
N
N
OH
OMe
Quinine (C lost)
N
NH CO2Me
Catharanthine
NH
N
CO2MeH
OAc
OH
Vindoline
Complextransformations
NH
NMe
HN
MeN
H
H
Chimonanthine
Cyclizationdimerization
Prenylationcyclization
NH
NMeHO2C
H
Lysergic acid
H
H
Ergot alkaloidsPyrroloindoles Indole monoterpene
1C lost
1C lostFrom AcCoA
SCHEME 18 Tryptophan‐derived alkaloid biosynthetic pathways (gray parts monoterpenic units)
10 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
different structure strychnine is related to the Corynanthe alkaloids incorporating two carbons from acetyl‐CoA Highly modified monoterpene skeletons are derived from the Corynanthe core through CC bond breaking and reorshyganization leading to Iboga‐type (eg catharanthine) and Aspidosperma‐type (eg vindoline) alkaloids The antishycancer drug vinblastine is a heterodimer resulting from the nucleophilic attack of vindoline on a mannich acceptor resulting from catharanthine found in madagascar perishywinkle (Catharanthus roseus) The heteroaromatic comshypounds ellipticine camptothecin and quinine are also derived from a Corynanthe‐type precursor although in this case the biosynthetic relationship may not be obvious due to deep modifications of the skeleton
Finally two important classes of compounds have to be mentioned since they have inspired many synthetic chemists The pyrroloindole alkaloids result from the cyclization of tryptamine as found in physostigmine (formed by a cationic mechanism after methylation in position 3 of the indole not shown) or in chimonanthine (presumably formed by a radical coupling mechanism Scheme 18) The ergot alkaloids are derived from the 33‐dimethylallylation on position 4 of the indole in trypshytophan whose further cyclization and oxidation processes afford the natural products (eg lysergic acid Scheme 18 and ergotamine) which have had important medical applications [34]
NRPS Metabolites and Peptides NRPS enzymes assemble amino acids including nonproteinogenic ones into oligopeptides The enzymes contain several modules and especially an adenylation domain (A) which specifically selects and activates the amino acid to be transferred as a thioester on the nearby peptidyl carrier protein (PCP) [2] A condensation module (C) then catalyzes the formation of the peptide bonds between the newly introduced amino acyl‐PCP (bearing a free amine) and the elongated peptidyl‐PCP thioester At the end of the elongation a cyclization can occur into cyclopeptides but the peptide can also be
transferred to auxiliary enzymes like methyltransferases glycosyltransferases or oxidases (vancomycins are typical products of such functionalizations) [35 36] The formation of heterocycles is also frequently encountered in this metabolism as in penicillins that are derived from the tripeptide α‐aminoadipoyl‐cysteinyl‐valine or telomestatin (Fig 14) [2]
Other Alkaloid Origins There are many other nitrogen sources involved in alkaloid biosyntheses for example nicotinic acid (originated from aspartic acid and intershymediate in nicotine and anabasine biosyntheses) and anthranilic acid (originated from tryptophan and intermedishyate toward acridines or aurachins) The amination reaction (eg through transamination of carbonyl compounds) is also a way to introduce nitrogens in natural products for example from fatty acids steroids (toward Solanum alkaloids or cyclopamines) or other terpenoids (aconitine and atisine have diterpene skeletons while Daphniphyllum alkaloids are triterpene derivatives) Finally nucleic acids can also be precursors of alkaloids like the well‐known caffeine
12 FROM BIOSYNTHESIS TO TOTAL SYNTHESIS STRATEGIES TOWARD THE NATURAL PRODUCT CHEMICAL SPACE
121 The Chemical Space of Natural Products
Natural products occupy an important place in human com munities as demonstrated by their vast use from ancient times to nowadays like dyes fibers oils pershyfumes agrochemicals or drugs Broadly both primary and secondary metabolites could be classified as natural products while the latter as discussed previously are usushyally regarded as the ldquonatural productsrdquo owing to their comshyplexity and diversity arising from a variety of biosynthetic pathways The structural chemical diversity found among
N
S
CO2HO
HNPhH2C
O
Penicillin G
OH
HN
HN
O
O
ONH2
O
OHO
NHMe
OCl
O
NH
NH
NH
O
ClHO
O
HN
H
H
HOOH
OHHO2C
Vancomycin aglycone
ON
NO
O
NN
O
O
N
N O
Me
S
N N
OMeH
Telomestatin
L-AAA-L-CYS-D-VAL
Enzymes
FIGURE 14 Structural diversity of nonribosomal peptide compounds (AAA α‐aminoadipic acid)
FROm BIOSyNTHESIS TO TOTAL SyNTHESIS STRATEGIES TOWARD THE NATuRAL PRODuCT CHEmICAL SPACE 11
all living organisms defining the chemical space of natural products [37] is the consequence of their evolution occurshyring as an adaptation of organisms to their environment It is commonly believed that secondary metabolites are proshyduced as messengers by living organisms or as weapons against enemies and thus they should have certain biological activities in a medicinal point of view [38] Indeed natural products are regarded as one of the main sources of medicines (Fig 15) From the traditional medicinal extracts to every single bioactive molecule the methods of extraction purification identification and biological investigation of natural products have been well established Their complex structures and interesting properties have attracted synthetic chemists to accomplish their total syntheses and that of medicinally relevant anashylogs sometimes in the industrial context [39] Thus tarshygeting the chemical space of natural products has never been more relevant than today Although the discovery of natural products demands time and labor‐consuming manipulations it is worth to notice that the knowledge on this chemical space is still continually growing while biological advances allow for discovering and undershystanding potential targets However increasing the chemical space of human‐made compounds based on natural products should benefit from transdisciplinary colshylaborations such as the use of coupled biosynthetic and chemical synthetic methods to design original ldquounnatural natural productsrdquo [40]
122 The Biosynthetic Pathways as an Inspiration for Synthetic Challenges
1221 Precursor‐Directed Biosyntheses and Mutashysynthetic Strategies to Increase the Chemical Space of Natural Products As the genetics and biochemistry of natural product biosynthesis are better understood novel biosynthetic techniques have been developed to study and generate new diversity in natural product analogs (Scheme 19) Precursor‐directed biosynthesis (PDB) is considered as the earliest example of combining chemical and biological methods for the generation of complex natural product analogs [41 42] This approach compared with the biosynthetic pathway of wild‐type metabolites (Scheme 19a) involves the feeding of analogs of the natural biosynthetic building blocks to the living organisms (Scheme 19b) usually bacteria or fungi which incorporate the modified precursors into the biosynthesized compound mutasynthesis also termed as mutational biosynthesis (mBS) involves the inactivation of a key step of the bioshysynthesis in a mutant microorganism (Scheme 19c) which can then be fed by various modified or advanced building blocks (mutasynthons Scheme 19d) [43] These mutasshyynthons could not be incorporated by the wild type due to specificities of the enzymatic machinery Build up on PDB mBS eliminates the natural biosynthetic intermediate thus generating a less complex mixture of metabolites and making the purification or yield of target products better
O NH
OHO
O
OO
O
O
OH
HOO
O O
OH
HOO
NH ONH2
O
ONH
NH
O
OCl Cl
O
O
OH
HN
HN
HN
HN
OO
HO
HN
OH
OHOH
O
O
HO
HO
OHO
O
OHH2N
O
OH3C
H
O
H
CH3
CH3
H
O O
NO
H
O
HO
O
NO
O
NH
HN
OHO
HONH
ON
H
HO
O
H2N
O OH
ONH
OH
ON OHO
OH
HO OS OH
OOOH
OHO
O
ON
OO
O
HO
O
O OH
OO
Micafunginantifungal
Rapamycinimmunosuppressive
Galantamineanti-Alzheimer
Taxolanticancer
Artemisininantimalarial
Vancomycinantibiotic
FIGURE 15 Some famous natural products currently used as drugs
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product
xvi PrEfACE
endeavor to complete this book I learned many things from their expertise but I also realized that only with tight collab-orations you can build long‐lasting friendships I would like to thank them all once again for their trust and effort to complete this book We all hope that the current work will contribute to a better understanding of the current status of
organic chemistry and to the discovery of novel strategies and tactics for the synthesis of natural products
Alexandros L ZografosSeptember 2015
Thessaloniki Greece
From Biosynthesis to Total Synthesis Strategies and Tactics for Natural Products First Edition Edited by Alexandros L Zografoscopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
11 FROM PRIMARY TO SECONDARY METABOLISM THE KEY BUILDING BLOCKS
111 Definitions
The primary and secondary metabolisms are traditionally distinguished by their distribution and utility in the living organism network Primary metabolites include carbohyshydrates lipids nucleic acids and proteins (or their amino acid constituents) and are shared by all living organisms on Earth They are transformed by common pathways which are studied by biochemistry (Fig 11) Secondary metabo-lites are structurally diverse compounds usually produced by a limited number of organisms which synthesize them for a special purpose like defense or signaling through specific biosynthetic pathways They are studied by natural product chemistry This distinction is not always so obvious and some compounds can be studied in the context of both primary and secondary metabolisms This is especially true nowadays with the use of genetic and biomolecular tools which tend to make natural product sciences more and more integrative However an important point to remember is that the primary metabolism furnishes key building blocks to the secondary metabolism It would be difficult to describe in detail the full biosynthetic pathshyways in this section We tried to organize the discussion as a vade mecum synthetically gathering information from extremely useful sources which will be cited at the end of this chapter
112 Energy Supply and Carbon Storing at the Early Stage of Metabolisms
The sunlight is essential to life except in some part of the deep oceans It provides energy for plant photosynthesis that splits molecules of water into protons and electrons and releases O
2 (Scheme 11) A proton gradient inside the plant
chloroplasts then drags a transmembrane ATP synthase comshyplex that produces adenosine triphosphate (ATP) while elecshytrons released from water are transferred to the coenzyme reducer nicotinamide adenine dinucleotide phosphate hydride (NADPH) A major function of chloroplasts is to fix CO
2 as a combination to ribulose‐15‐bisphosphate (RuBP)
performed by RuBP carboxylase (rubisco) forming an instable ldquoC
6rdquo β‐ketoacid This is cleaved into two molecules
of 3‐phosphoglycerate (3‐PGA) which is then reduced into 3‐phosphoglyceraldehyde (3‐PGAL a ldquoC
3rdquo triose phosshy
phate) during the Calvin cycle This is one of the major metabolites in the biosynthesis of carbohydrates like glucose and a biochemical mean for storing and retaining carbon atoms in the living cells
113 Glucose as a Starting Material Toward Key Building Blocks of the Secondary Metabolism
Glucose‐6‐phosphate arises from the phosphorylation of glucose It is the starting material of glycolysis an important process of the primary metabolism which consists in eight enzymatic reactions leading to pyruvic acid (PA)
FROM BIOSYNTHESES TO TOTAL SYNTHESES AN INTRODUCTION
Bastien Nay1 and Xu‐Wen Li2
1
1 Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France2 Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
2 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
(Scheme 12) Important intermediates for the secondary metabolism are produced during glycolysis Glucose glucose‐6‐phosphate and fructose‐6‐phosphate can be converted to other hexoses and pentoses that can be oligoshymerized and enter in the composition of heterosides Additionally fructose‐6‐phosphate connects the pentose
phosphate pathway leading to erythrose‐4‐phosphate toward shikimic acid which is a key metabolite in the biosynthesis of aromatic amino acids (phenylalanine tyrosine or C
6C
3
units) and C6C
1 phenolic compounds The next important
intermediate in glycolysis is 3‐PGAL which can be redishyrected toward methylerythritol‐4‐phosphate (mEP) in the
Primary metabolism Secondary metabolism
The field ofbiochemistry
The field ofnatural product
chemistry
Essential toliving organisms
Essential to theproducer organisms
under particularconditions
Nucleic acids (DNARNA) carbohydrates
lipids amino acidspeptides and proteins
Alkaloids terpenespolyketides
polyphenols andtheir heterosidic form
Biological effects(defense signaling)
Biosyntheticpathways
Main compoundclasses
FIGURE 11 Primary versus secondary metabolisms
PS-I
PS-IIendash
hν H2O H+ and O2
ATP synthase
NADP reductase
H+ (gradient)
H+ADP + Pi ATP
NADPHH+NADP+
H+
Thylakoidcompartment
Chloroplaststroma
Thylakoidmembrane
(a) Light dependent process
(b) Light independent process
CO2
RuBP
Rubisco
Unstable β-ketoacid3-PGA 13-diPGA
ATP
ADP3-PGAL
NADPHH+
NADP+
(Calvin cycle)
trioses tetroses pentoses hexoses (eg glucose) heptoses
Chloroplaststroma
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
CH2OPO32ndash CH2OPO3
2ndash CH2OPO32ndash
CH2OPO32ndash
OOH
HO
HOO
HO
HO2CCO2H
HO
CO2PO32ndash
HO
CHOHO
endash
Cytosol
SCHEME 11 The photosynthetic machinery (PS‐I and PS‐II photosystems I and II)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 3
chloroplast mEP is a starting block in the biosynthesis of terpenes through C
5 isoprene units (isopentenyl diphosphate
(IPP) and dimethylallyl diphosphate (DmAPP)) especially those in C
10 C
20 and C
40 terpenes 3‐PGA is a precursor of
serine and other amino acids while phosphoenolpyruvate (PEP) the precursor of PA is also an intermediate toward the previously mentioned shikimic acid Lastly PA is not only a precursor of the fundamental ldquoC
2rdquo acetyl coenzyme A
(AcCoA) unit but also an intermediate toward aliphatic amino acids and mEP
AcCoA is the building block of fatty acids polyketides and mevalonic acid (mVA) a cytosolic precursor of the C
5 isoprene units for the biosynthesis of terpenes in the
C15
and C30
series (mind it is different from the mEP pathway in product and in cell location) Finally AcCoA enters the citric acid or Krebs cycle which leads to several precursors of amino acids These are oxaloacetic acid precursor of aspartic acid through transamination (thus toward lysine as a nitrogenated C
5N linear unit and methishy
onine as a methyl supplier) and 2‐oxoglutaric acid preshycursor of glutamic acid (and subsequent derivatives such as ornithine as a nitrogenated C
4N linear unit) All these
amino acids are key precursors in the biosynthesis of many alkaloids
114 Reactions Involved in the Construction of Secondary Metabolites
most reactions occurring in the living cells are performed by specialized enzymes which have been classified in an intershynational nomenclature defined by an enzyme commission (EC) number There are six classes of enzymes depending on the biochemical reaction they catalyze EC‐1 oxidoreducshytases (catalyzing oxidoreduction reactions) EC‐2 transfershyases (catalyzing the transfer of functional groups) EC‐3 hydrolases (catalyzing hydrolysis) EC‐4 lyases (breaking bonds through another process than hydrolysis or oxidation leading to a new double bond or a new cycle) EC‐5 isomershyases (catalyzing the isomerization of a molecule) and EC‐6 ligases (forming a covalent bond between two molecules) many subclasses of these enzymes have been described depending on the type of atoms and functional groups involved in the reaction and if any on the cofactor used in this reaction For example several cofactors can be used by dehydrogenases like NAD(P)NAD(P)H FADFADH
2 or
FmNFmNH2 For a description of this classification the
reader can refer to specialized Internet websites like ExplorEnz [1] What is important to realize is that most enzymes are substrate specific and have been selected during
CHOHO
CO2H
O
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
HOOH
HO
HOH2CO
OHC
CH2OPO32ndash
OHHO
CO2H
HO
OH
OH
TYR
PHE
CO2H
NH2
TRP
CH2OPO32ndash
HO
HOH2COH
MEP
C10 C20 and C40 terpenes
CO2HHO
SER
GLY CYS
CO2HOPO3
2ndash
CH2OH
OHHO2C
MVA
C15 and C30 terpenes
Polyketides
Krebscycle
(citric acid)
VAL ALA ILE LEU
CO2H
O
HO2C
CO2HO
CO2H
ASP
LYS
GLU
Glucose-6-phosphate
Fructose-6-phosphate
Erythrose-4-phosphate
3-PGAL
OP2O63ndash OP2O6
3ndash
IPP DMAPP
3-PGA PEP PA
O SCoAAcCoA
3x
Cytosol
Cytosol
Chloroplasts
Chloroplasts
Glycolysis
C2
C5
Shikimic acid Oxaloaceticacid
2-Oxoglutaricacid
MET
THR
PRO
ARGORN
Alkaloids Alkaloids Alkaloids Alkaloids
Aminoacids
Peptidesproteins
Phenolics
C1
C5N C4N
ldquoCH3rdquo
(Indole)C2NC6C3C6C2N C6C1
Pentosephosphatepathway
SCHEME 12 The building block chart involving glycolysis and the Krebs cycle
4 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
evolution to perform specific transformations making natural products with often and yet unknown functions
Secondary metabolites arise from specific biosynthetic pathways which use the previously defined building blocks The bunch of organic reactions involved in these biosynshytheses allows the construction of natural product frameshyworks which are finally diversified through ldquodecorationrdquo steps (Scheme 13) It is not the purpose of this introductive chapter to describe in detail all biosynthetic pathways and the reader can refer to excellent books and articles which have been published elsewhere [2 3]
The reactions involved in the construction of natural product skeletons will be described later for representative classes of compounds The identification of the building block footprint in the natural product skeleton will be emphasized as much as possible sometimes referring to biogenetic speculations [4] After the framework construction the decoration steps will involve as diverse reactions as aliphatic CH oxidations (eg involving a cytochrome P
450 oxygenase) occasionally triggering a
rearrangement heteroatom alkylations (eg methylation by S‐adenosylmethionine) or allylation (by DmAPP) esterifications heteroatom or C‐glycosylations (leading to heterosides) radical couplings (especially for phenols) alcohol oxidations or ketone reductions amineketone transaminations alkene dihydroxylations or epoxidations oxidative halogenations BaeyerndashVilliger oxidations and further oxygenation steps At the end of the biosynthesis such transformations may totally hide the primary building block origin of natural products
115 Secondary Metabolisms
1151 Polyketides Polyketides (or polyacetates) are issued from the oligomerization of C
2 acetate units performed
by polyketide synthases (PKS) and leading to (C2)
n linear
intermediates [5 6] If the (C2)
n intermediates arise from
successive Claisen reactions performed by ketosynthase
domains (KS in nonreducing PKS) a highly reactive poly‐ β‐ketoacyl intermediate H(CH
2CO)
nOH is formed
leading to phenolic and aromatic products through further intramolecular Claisen condensations Furthermore highly reducing PKSs are made of specialized enzymatic subunits working in line or iteratively to functionalize each C
2 linker
bond as CH(OH)CH2 (by ketoreductases (KR)) then as
HCCH (by dehydratases (DH)) and as CH2CH
2 (by enoyl
reductases (ER)) leading to a high degree of functionalization of the final product (Fig 12) By these iterative sequences highly reduced polyketides which can be either linear macrocyclized or polycyclized depending on the reactivity of the biosynthetic intermediates can be formed [7] With the same logic fatty acids are also biosynthesized by fatty acid synthases
moreover the PKS enzyme can be hybridized with nonshyribosomal peptide synthetase (NRPS) domains (see also ldquoNRPS metabolites and peptidesrdquo in the ldquoAlkaloidsrdquo secshytion) leading to the acylation of an amino acid by the (C
2)
n
acyl intermediate As previously the functionalization of the acyl chain depends on the PKS enzyme and the PKSNRPS products are also extremely diversified (eg hirsutellone B Fig 12) [8]
1152 Terpenes Terpenes are derived from the oligoshymerization of the C
5 isoprene units DmAPP and IPP Both
precursors are prompt to generate either an allylic cation (the diphosphate is a good leaving group) or a tertiary carbocation respectively which makes the IPP easy to react with DmAPP (Scheme 14) This reaction happens in the active site of a terpene synthase which activates the deparshyture of the diphosphate group from DmAPP thanks to Lewis acid activation (a metal like mg2+ mn2+ or Co2+ is present in the enzyme active site [9]) This leads to geranyl (C
10
monoterpene precursor) or farnesyl (C15
sesquiterpene precursor) diphosphate depending on the location of the enzyme (chloroplast for the mEP pathway or cytosol for the mVA pathway) Geranylgeranyl (C
20 diterpene) and
Constructionreactions
Decoration reaction(functionalization)Building
blocksNatural product
frameworksNatural products
HH
OH OAcH
HO O OH
OHO
OBz
P450
P450
P450
P450
P450 P450 Oxidativeauxiliaryenzymes
Taxadiene
OP2O63ndash
OP2O63ndash
3times
(a)
(b)
10-Deacetylbaccatin III
DMAPP
IPPand electrophilic
cyclizations
SCHEME 13 (a) From building blocks to natural products and (b) the example of 10‐deacetylbaccatin III
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 5
farnesylfarnesyl (C30
triterpene precursor) diphosphates can also be obtained by further additions of IPP leading to longer linear intermediates
The cyclization of linear precursors is achieved by speshycialized cyclases which generate a poorly functionalized natural product framework [10 11] Auxiliary enzymes such as oxygenases then increase the complexity and the diversity
of compounds by further functionalization (Scheme 13b) [12] A high degree of oxidation can be observed in comshypounds like thapsigargin paclitaxel or bilobalide (Fig 13) The biosynthesis of this last compound for example involves a high oxygenation pattern two Wagnerndashmeerwein rearrangements and several oxidative cleavages leading to the loss of five carbons The resulting natural products can
KS
S-ACP
O O
KR
S-ACP
OH O
S-MAT
O
YesNo
R
R
R
YesNoDH
S-ACP
OR
YesNoER
S-ACP
OR
YesNoTE
OH
OR
New cycle
New cycleor
release
New cycleor
release
New cycleor
release
Release
R in
crem
ente
d by
two
carb
ons
HO
O
S-ACP
O
Claisen condensation (ndashCO2)
reduction (NADPH)
dehydration (ndashH2O)
+
reduction (NADPH)
hydrolysis (H2O)
O O NH
OH
OH
H H
H H
Hirsutellone B(mixed PKSNRPS
product)
OO
O
OHOH
HO OH
Norsolorinic acid
O
O
O
O CHO
OH
HH
H
H
HH
OHHemibrevetoxin B
OOHO
OHO
HOHO OH
Erythronolide A
OH
OH
HO
Panaxytriol
From ahexanoylstartingblock
H
H
Fromtyrosine
H
1C lost fromdecarboxylation
FIGURE 12 Chemical logic of polyketide construction leading to variable functionalization of the elongated acyl chain and examples of resulting chemical diversity ACP acyl carrier protein DH dehydratase ER enoyl reductase KR ketoreductase KS ketosynthase mAT malonyl acyl transferase TE thioesterase
DMAPPOP2O6
3ndashOP2O6
3ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
Geranyl diphosphate (GPP)2 times IPP
H H
Geranylgeranyl diphosphate (GGPP)
Examplesverticillane
casbanetaxane
phorbol
Exampleslabdane
pimaranekauraneabietane
aphidicolanegibberellane
IPP
SCHEME 14 Early assembly of C5 units in terpene biosynthesis leading to diterpenes (C
20)
6 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
thus be extremely modified with structures whose biogenetic origin is far from being obvious at first sight and cannot be determined without further experiments such as isotopic labeling
1153 Flavonoids Resveratrols Gallic Acids and Further Polyphenolics We have previously discussed the polyketide origin of some phenolic compounds based on the (C
2)
n motif Other polyphenols like gallic acids are directly
derived by the aromatization of shikimic acid (C6C
1 building
block Scheme 12) [13] The C6C
3 building blocks are availshy
able from the conversion of phenylalanine and tyrosine into cinnamic and p‐coumaric acids respectively and then by further hydroxylation steps (Scheme 15) These can dimerize into lignans (eg podophyllotoxin) [14 15] through radical processes or converted to low molecular weight compounds like eugenol coumarins or vanillin [16] The coenzyme A thioesters of these C
6C
3 acids can be used as initiator units
by specialized ketosynthases for an elongation by two acetyl units leading to aromatic polyketides like styrylpyrones or diarylheptanoids (eg curcumin) [17] Important comshypounds from this metabolism are flavonoids (C
6C
3C
6) [18]
and stilbenoids (C6C
2C
6) (a decarboxylation occurs during
the aryl cyclization) [19] which are synthesized by chalcone synthase and stilbene synthase respectively Flavonoids (eg catechin) and stilbenes (eg resveratrol) are present in large amounts in fruits and vegetables and may exert their radical scavenging properties in vivo
1154 Alkaloids Alkaloids are nitrogen‐containing compounds The nitrogen(s) can be involved in an amine function conferring basicity to the natural product (like ldquoalkalirdquo) or in less or nonbasic functions such as an amide a nitrile an isonitrile or an ammonium salt (quaternary amines) For amines protonation often occurs at physiological pH and may condition their biological activity In many cases the nitrogen is biogenetically derived from an amino acid We will thus discuss alkaloids according to their amino acid origin
Alkaloids Derived from the Krebs Cycle (Lysine and Ornithine Derived) As shown previously (Scheme 12) the Krebs cycle is a crucial metabolic process which leads to α‐ketoacids (oxaloacetic and 2‐oxoglutaric acids) Their enzymatic transamination affords the two amino acidsmdashaspartic acid and glutamic acidmdashwhich are the direct biosynthetic precursors of amino acids lysine and ornithine respectively These in turn produce cadaverine a ldquoC
5Nrdquo unit
and putrescine a ldquoC4Nrdquo unit which are major components
for the biosynthesis of important alkaloids as will be discussed later (Scheme 16) Additionally ornithine is a precursor of arginine another important amino acid
ornithine‐derived alkaloids (incorporating the c4n
unit) Putrescine is derived from the decarboxylation of ornithine and is a precursor of linear polyamines like spermshyine After enzymatic methylation of one amine of putrescine
C10 C15
C15
C10
C10
C30
CO2H
Chrysanthemic acid
OH
Menthol
O
HO
H
HOGlc
MeO2CLoganin (iridoid)
H
H
H
HH
OO
OParthenolide
O
O
OHOHO
OAcHO
OnC3H7
O
O
O
nC7H15
Thapsigargin
O
O
OO
H
H
O
Cleavage
H
H
Artemisinin
C20 C40
O
O
OO
OO
OHOHH
Bilobalide(highly rearrangedand missing 5 C)
CleavageH
Highoxidativecleavages
HO OAcH
AcO O OH
OOOBz
O
Ph
BzNH
OHFrom PHE Paclitaxel
C25O
H
HO
OHCH
OH Ophiobolin A
H
HO
H
H
HHCholesterol
(missing 3 C WM)
H
H H H
H
H
Hopene
β-Carotene
C30 - 3
C15
C20 - 5WM
WM
FIGURE 13 Chemical diversity in the terpene series (Wm Wagnerndashmeerwein shifts lost carbons bold bonds are remnant of primary building blocks)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 7
in the presence of S‐adenosylmethionine transamination of the other affords γ‐(N‐methylamino)aldehyde [20] The resulting cyclic iminium is a key intermediate in the formation of many medicinally important alkaloids such as
the plant‐derived compounds cocaine atropine or the calysshytegines [21 22] Indeed this iminium is a mannich acceptor which can react with various nucleophiles the first of those being the carbanion of acetyl‐CoA Thus after a stepwise
CO2H
RCinnamic acid (R=H)p-Coumaric acid (R=OH)
RO
PHETYR
OH
[H] [O]
lignans
OH
O
O
O
O
OMeMeO
OMe
C mdash C andor C mdash Oradical couplings
After E Zisomerization
for example Podophyllotoxin
O ORO
Coumarins(eg scopoletin)
[O]
Prenylationcyclizationcleavage[O]
O OO
Furocoumarins(eg psoralen)
RO
nAcetyl-CoA
thencyclization
n = 2 styrylpyrones
O O
OMe
n = 3 avonoids(from chalcone synthase)
C6C3
H
H H
From AcCoA
O
n = 3 stilbenoids(from stilbene synthase)
HO
OH
OHFrom AcCoA From AcCoA(ndashCO2)
Resveratrol
HO
OH
OH
OH
OH
H
Catechin
MeO Yangonin
From AcCoA
SCHEME 15 The phenylpropanoid biosynthetic pathways
LYS
CO2
Cadaverine
O NH
H2O
NH
Piperideineiminium
Pelletierine
AcAcCoA
CO2
NH
O
N
O
Pseudopelletierine
NH
Ph
OH
Ph
O
Lobeline
NH
δ+
δndash
N
H
H
N
H
OH
Lupinine
N
H N
H
(+)-Sparteine
N
NH
O (ndash)-Cytisine
NH
CO2H
Pipecolic acid
N
OHHO
OH
HO
H
Castanos permine
ORN
CO2
NH2 NH2 NH2 NH2
NH2
NH2
NH2
PutrescineO
NHMe
N-Methylpyrrolinium
2 timesAcCoA
NMe
ARG
n = 1 spermidinen = 2 homospermidine
NMe
O
HygrineCO2
CO2
MeN
O
[O]
TropinoneCO2
HNHO
OHOH
OH
Calystegine B2
MeN
OAtropine
O
Ph
HO
NMe
NMe
NMe
O
Cuscohygrine
HN
HN
NH2
( )n
O
n = 2
N
HHO
Retronecine
HH
H
H
H
HO
H H
H
SCHEME 16 Lysine‐ and ornithine‐derived alkaloid biosynthetic pathways (mind the structural similarities)
8 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
elongation by two AcCoA units either decarboxylation can occur leading to the acetonylpyrrolidine hygrine or a secshyond mannich reaction by the intramolecular attack of the acetoacetate anion onto an oxidation‐derived pyrrolinium leading to the tropane skeleton (tropinone) The acetoacetyl‐CoA intermediate can also react intermolecularly with another pyrrolinium cation leading to cuscohygrine after decarboxylation Finally the pyrrolizidine alkaloids [23] are derived from homospermidine which when submitted to terminal oxidative deamination leads to the bicyclic skelshyeton of retronecine and further Senecio alkaloids We can mention herein that ornithine is a biosynthetic precursor of arginine bearing a guanidine function which is an intermediate toward the toxic compounds tetrodotoxin and saxitoxin (not shown)
lysine‐derived alkaloids (incorporating the c5n
unit) From lysine to piperidine alkaloids the biosynthetic steps parallel the one previously described from ornithine Indeed the oxidative deamination of cadaverine affords a δ‐amino aldehyde which cyclizes through imine formation into piperideine Protonation results in a mannich acceptor which is able to react with various nucleophiles such as β‐ketothioester anions The first product of these reactions is pelletierine which can further react through an intramolecular mannich
reaction leading to pseudopelletierine Quinolizidines [24] can also be formed first from the mannich reaction of the piperideine acceptor with the corresponding enamine nucleoshyphile and then after additional transformation steps leading for example to lupinine sparteine or cytisine
Indolizidine alkaloids [15] such as castanospermine and swainsonine are formed from pipecolic acid an amino acid derived from lysine which can be elongated by malonyl‐CoA followed by ring closure When protonated these alkaloids are oxonium mimics strongly inhibiting glycosidases
Tyrosine‐ and Phenylalanine‐Derived Alkaloids Tyrosine and phenylalanine amino acids are bearing the phenylshyethylamine moiety of many medicinally relevant alkaloids Further hydroxylations on the aromatic carbocycle or on the aliphatic part can be observed methylations can occur on phenolic oxygens and on the amine leading to catecholamines (adrenaline noradrenaline dopamine) Arylethylamines are also usual to react with endogenous aldehydes through PictetndashSpengler reactions [25] leading to important biosynthetic intermediates (Scheme 17) like
bull Reticuline from the reaction with 4‐hydroxyphenylacetshyaldehyde toward benzyltetrahydroisoquinoline alkaloids
NH2
Phenylethylamines
RONH TetrahydroisoquinolinesRO
RʹCHO
Rʹ
NH
(CH2)n
MeO
HO
Benzyltetrahydroisoquinoline(n = 1) for example reticuline
orPhenethyltetrahydroisoquinoline
(n = 2) eg autumnaline
IntermolecularCmdashO phenol
couplings
Curarealkaloids
IntramolecularC mdash C phenol
couplings
ONMe
HHO
H
HO
Morphine
NMe
MeO
HO
MeOOH
Isoboldine
O
O
OMe
NO2
CO2H
Aristolochic acid
OMe
O
NHAcMeO
MeOMeO
Colchicine
N
O
O
OMe
OMeBerberine
IntramolecularCmdash C coupling
and furtheroxidative events
Rʹ derivedfrom PHE
Pictet-Spenglerreaction
TYR
HO
HO CHO
HNHO
HO
OH
OMeO
OH
NMe
[O]
GalantamineNorbelladine
Rʹ derived fromsecologanin
NAc
HO
HO
O
CO2MeH
H
H
OHIpecosideaglycone
Ipecac alkaloids
1ClostCmdash C cleavage
and ring extension
H
ROH
1C lost
Cleavage
SCHEME 17 Tyrosine‐derived alkaloid biosynthetic pathways (double head arrows figure bond cleavages during biosynthetic processes)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 9
morphine berberine tubocurarine isoboldine or the highly modified aristolochic acid [26 27]
bull Automnaline from the reaction with 3‐(4‐hydroxyphenyl)propanal toward phenylethyltetrahydroisoquinoline alkaloids colchicine cephalotaxine or schelhammerishycine [28 29]
bull Ipecoside from the reaction of dopamine with secoloshyganin toward terpene tetrahydroisoquinoline alkaloids ipecoside or emetine
Lastly norbelladine (top of Scheme 17) is issued from the reductive amination of 34‐dihydroxybenzaldehyde (derived from phenylalanine) with tyramine (derived from tyrosine) and constitutes a biosynthetic node leading to Amaryllidaceae alkaloids such as galantamine crinine or lycorine depending on the topology of phenolic couplings In all these biosynthetic routes radical phenolic couplings are key reactions for CC and CO bond formations and rearrangements [30 31]
Tryptophan‐Derived Indole and Indole Monoterpene Alkaloids As for alkaloids derived from tyrosine and phenylalanine those derived from tryptophan are formed after decarboxylation of the amino acid (into tryptamine) and possible hydroxylation of the aromatic carbocycle (eg serotonin) and N‐methylation (eg psilocin) As previously tryptamine can also react through PictetndashSpengler reactions to form tetrahydro‐β‐carbolines which can be aromatized for example into harmine (Scheme 18) [16]
When the aldehyde partner of the PictetndashSpengler reacshytion with tryptamine is the terpene secologanin strictosidine is formed as an entry toward the vast monoterpene indole alkaloids [32 33] Hydrolysis of the glucosidic part releases the strictosidine aglycone bearing an aldehyde while iminshyium formation and further cyclization and reduction can lead to ajmalicine (from oxocyclization) or yohimbine (from carshybocyclization) These alkaloids are referred to as from the Corynanthe type with the monoterpene carbon skeleton unmodified Although it misses one carbon and has a very
RO
Carbonylcompounds
TRPNH
NH2
Indolethylamines(eg serotonine)
RONH
NH
Rʹ NH
N
MeOPictet-Spenglerreaction for example Harmineβ-Carbolines
NH
NHH
O
MeO2C
MeO2C
OH
H
H
Rʹ derived from secologanin
Strictosidine aglycone
NH
N
OH
H
H
Ajmalicine
N
N
O
O
H
H
H
Strychnine (C lost)Corynanthe type
Aspidosperma type Iboga type
N
N
OH
OMe
Quinine (C lost)
N
NH CO2Me
Catharanthine
NH
N
CO2MeH
OAc
OH
Vindoline
Complextransformations
NH
NMe
HN
MeN
H
H
Chimonanthine
Cyclizationdimerization
Prenylationcyclization
NH
NMeHO2C
H
Lysergic acid
H
H
Ergot alkaloidsPyrroloindoles Indole monoterpene
1C lost
1C lostFrom AcCoA
SCHEME 18 Tryptophan‐derived alkaloid biosynthetic pathways (gray parts monoterpenic units)
10 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
different structure strychnine is related to the Corynanthe alkaloids incorporating two carbons from acetyl‐CoA Highly modified monoterpene skeletons are derived from the Corynanthe core through CC bond breaking and reorshyganization leading to Iboga‐type (eg catharanthine) and Aspidosperma‐type (eg vindoline) alkaloids The antishycancer drug vinblastine is a heterodimer resulting from the nucleophilic attack of vindoline on a mannich acceptor resulting from catharanthine found in madagascar perishywinkle (Catharanthus roseus) The heteroaromatic comshypounds ellipticine camptothecin and quinine are also derived from a Corynanthe‐type precursor although in this case the biosynthetic relationship may not be obvious due to deep modifications of the skeleton
Finally two important classes of compounds have to be mentioned since they have inspired many synthetic chemists The pyrroloindole alkaloids result from the cyclization of tryptamine as found in physostigmine (formed by a cationic mechanism after methylation in position 3 of the indole not shown) or in chimonanthine (presumably formed by a radical coupling mechanism Scheme 18) The ergot alkaloids are derived from the 33‐dimethylallylation on position 4 of the indole in trypshytophan whose further cyclization and oxidation processes afford the natural products (eg lysergic acid Scheme 18 and ergotamine) which have had important medical applications [34]
NRPS Metabolites and Peptides NRPS enzymes assemble amino acids including nonproteinogenic ones into oligopeptides The enzymes contain several modules and especially an adenylation domain (A) which specifically selects and activates the amino acid to be transferred as a thioester on the nearby peptidyl carrier protein (PCP) [2] A condensation module (C) then catalyzes the formation of the peptide bonds between the newly introduced amino acyl‐PCP (bearing a free amine) and the elongated peptidyl‐PCP thioester At the end of the elongation a cyclization can occur into cyclopeptides but the peptide can also be
transferred to auxiliary enzymes like methyltransferases glycosyltransferases or oxidases (vancomycins are typical products of such functionalizations) [35 36] The formation of heterocycles is also frequently encountered in this metabolism as in penicillins that are derived from the tripeptide α‐aminoadipoyl‐cysteinyl‐valine or telomestatin (Fig 14) [2]
Other Alkaloid Origins There are many other nitrogen sources involved in alkaloid biosyntheses for example nicotinic acid (originated from aspartic acid and intershymediate in nicotine and anabasine biosyntheses) and anthranilic acid (originated from tryptophan and intermedishyate toward acridines or aurachins) The amination reaction (eg through transamination of carbonyl compounds) is also a way to introduce nitrogens in natural products for example from fatty acids steroids (toward Solanum alkaloids or cyclopamines) or other terpenoids (aconitine and atisine have diterpene skeletons while Daphniphyllum alkaloids are triterpene derivatives) Finally nucleic acids can also be precursors of alkaloids like the well‐known caffeine
12 FROM BIOSYNTHESIS TO TOTAL SYNTHESIS STRATEGIES TOWARD THE NATURAL PRODUCT CHEMICAL SPACE
121 The Chemical Space of Natural Products
Natural products occupy an important place in human com munities as demonstrated by their vast use from ancient times to nowadays like dyes fibers oils pershyfumes agrochemicals or drugs Broadly both primary and secondary metabolites could be classified as natural products while the latter as discussed previously are usushyally regarded as the ldquonatural productsrdquo owing to their comshyplexity and diversity arising from a variety of biosynthetic pathways The structural chemical diversity found among
N
S
CO2HO
HNPhH2C
O
Penicillin G
OH
HN
HN
O
O
ONH2
O
OHO
NHMe
OCl
O
NH
NH
NH
O
ClHO
O
HN
H
H
HOOH
OHHO2C
Vancomycin aglycone
ON
NO
O
NN
O
O
N
N O
Me
S
N N
OMeH
Telomestatin
L-AAA-L-CYS-D-VAL
Enzymes
FIGURE 14 Structural diversity of nonribosomal peptide compounds (AAA α‐aminoadipic acid)
FROm BIOSyNTHESIS TO TOTAL SyNTHESIS STRATEGIES TOWARD THE NATuRAL PRODuCT CHEmICAL SPACE 11
all living organisms defining the chemical space of natural products [37] is the consequence of their evolution occurshyring as an adaptation of organisms to their environment It is commonly believed that secondary metabolites are proshyduced as messengers by living organisms or as weapons against enemies and thus they should have certain biological activities in a medicinal point of view [38] Indeed natural products are regarded as one of the main sources of medicines (Fig 15) From the traditional medicinal extracts to every single bioactive molecule the methods of extraction purification identification and biological investigation of natural products have been well established Their complex structures and interesting properties have attracted synthetic chemists to accomplish their total syntheses and that of medicinally relevant anashylogs sometimes in the industrial context [39] Thus tarshygeting the chemical space of natural products has never been more relevant than today Although the discovery of natural products demands time and labor‐consuming manipulations it is worth to notice that the knowledge on this chemical space is still continually growing while biological advances allow for discovering and undershystanding potential targets However increasing the chemical space of human‐made compounds based on natural products should benefit from transdisciplinary colshylaborations such as the use of coupled biosynthetic and chemical synthetic methods to design original ldquounnatural natural productsrdquo [40]
122 The Biosynthetic Pathways as an Inspiration for Synthetic Challenges
1221 Precursor‐Directed Biosyntheses and Mutashysynthetic Strategies to Increase the Chemical Space of Natural Products As the genetics and biochemistry of natural product biosynthesis are better understood novel biosynthetic techniques have been developed to study and generate new diversity in natural product analogs (Scheme 19) Precursor‐directed biosynthesis (PDB) is considered as the earliest example of combining chemical and biological methods for the generation of complex natural product analogs [41 42] This approach compared with the biosynthetic pathway of wild‐type metabolites (Scheme 19a) involves the feeding of analogs of the natural biosynthetic building blocks to the living organisms (Scheme 19b) usually bacteria or fungi which incorporate the modified precursors into the biosynthesized compound mutasynthesis also termed as mutational biosynthesis (mBS) involves the inactivation of a key step of the bioshysynthesis in a mutant microorganism (Scheme 19c) which can then be fed by various modified or advanced building blocks (mutasynthons Scheme 19d) [43] These mutasshyynthons could not be incorporated by the wild type due to specificities of the enzymatic machinery Build up on PDB mBS eliminates the natural biosynthetic intermediate thus generating a less complex mixture of metabolites and making the purification or yield of target products better
O NH
OHO
O
OO
O
O
OH
HOO
O O
OH
HOO
NH ONH2
O
ONH
NH
O
OCl Cl
O
O
OH
HN
HN
HN
HN
OO
HO
HN
OH
OHOH
O
O
HO
HO
OHO
O
OHH2N
O
OH3C
H
O
H
CH3
CH3
H
O O
NO
H
O
HO
O
NO
O
NH
HN
OHO
HONH
ON
H
HO
O
H2N
O OH
ONH
OH
ON OHO
OH
HO OS OH
OOOH
OHO
O
ON
OO
O
HO
O
O OH
OO
Micafunginantifungal
Rapamycinimmunosuppressive
Galantamineanti-Alzheimer
Taxolanticancer
Artemisininantimalarial
Vancomycinantibiotic
FIGURE 15 Some famous natural products currently used as drugs
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product
From Biosynthesis to Total Synthesis Strategies and Tactics for Natural Products First Edition Edited by Alexandros L Zografoscopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
11 FROM PRIMARY TO SECONDARY METABOLISM THE KEY BUILDING BLOCKS
111 Definitions
The primary and secondary metabolisms are traditionally distinguished by their distribution and utility in the living organism network Primary metabolites include carbohyshydrates lipids nucleic acids and proteins (or their amino acid constituents) and are shared by all living organisms on Earth They are transformed by common pathways which are studied by biochemistry (Fig 11) Secondary metabo-lites are structurally diverse compounds usually produced by a limited number of organisms which synthesize them for a special purpose like defense or signaling through specific biosynthetic pathways They are studied by natural product chemistry This distinction is not always so obvious and some compounds can be studied in the context of both primary and secondary metabolisms This is especially true nowadays with the use of genetic and biomolecular tools which tend to make natural product sciences more and more integrative However an important point to remember is that the primary metabolism furnishes key building blocks to the secondary metabolism It would be difficult to describe in detail the full biosynthetic pathshyways in this section We tried to organize the discussion as a vade mecum synthetically gathering information from extremely useful sources which will be cited at the end of this chapter
112 Energy Supply and Carbon Storing at the Early Stage of Metabolisms
The sunlight is essential to life except in some part of the deep oceans It provides energy for plant photosynthesis that splits molecules of water into protons and electrons and releases O
2 (Scheme 11) A proton gradient inside the plant
chloroplasts then drags a transmembrane ATP synthase comshyplex that produces adenosine triphosphate (ATP) while elecshytrons released from water are transferred to the coenzyme reducer nicotinamide adenine dinucleotide phosphate hydride (NADPH) A major function of chloroplasts is to fix CO
2 as a combination to ribulose‐15‐bisphosphate (RuBP)
performed by RuBP carboxylase (rubisco) forming an instable ldquoC
6rdquo β‐ketoacid This is cleaved into two molecules
of 3‐phosphoglycerate (3‐PGA) which is then reduced into 3‐phosphoglyceraldehyde (3‐PGAL a ldquoC
3rdquo triose phosshy
phate) during the Calvin cycle This is one of the major metabolites in the biosynthesis of carbohydrates like glucose and a biochemical mean for storing and retaining carbon atoms in the living cells
113 Glucose as a Starting Material Toward Key Building Blocks of the Secondary Metabolism
Glucose‐6‐phosphate arises from the phosphorylation of glucose It is the starting material of glycolysis an important process of the primary metabolism which consists in eight enzymatic reactions leading to pyruvic acid (PA)
FROM BIOSYNTHESES TO TOTAL SYNTHESES AN INTRODUCTION
Bastien Nay1 and Xu‐Wen Li2
1
1 Museacuteum National drsquoHistoire Naturelle and CNRS (UMR 7245) Uniteacute Moleacutecules de Communication et Adaptation des Microorganismes Paris France2 Shanghai Institute of Material Medica Chinese Academy of Science Shanghai China
2 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
(Scheme 12) Important intermediates for the secondary metabolism are produced during glycolysis Glucose glucose‐6‐phosphate and fructose‐6‐phosphate can be converted to other hexoses and pentoses that can be oligoshymerized and enter in the composition of heterosides Additionally fructose‐6‐phosphate connects the pentose
phosphate pathway leading to erythrose‐4‐phosphate toward shikimic acid which is a key metabolite in the biosynthesis of aromatic amino acids (phenylalanine tyrosine or C
6C
3
units) and C6C
1 phenolic compounds The next important
intermediate in glycolysis is 3‐PGAL which can be redishyrected toward methylerythritol‐4‐phosphate (mEP) in the
Primary metabolism Secondary metabolism
The field ofbiochemistry
The field ofnatural product
chemistry
Essential toliving organisms
Essential to theproducer organisms
under particularconditions
Nucleic acids (DNARNA) carbohydrates
lipids amino acidspeptides and proteins
Alkaloids terpenespolyketides
polyphenols andtheir heterosidic form
Biological effects(defense signaling)
Biosyntheticpathways
Main compoundclasses
FIGURE 11 Primary versus secondary metabolisms
PS-I
PS-IIendash
hν H2O H+ and O2
ATP synthase
NADP reductase
H+ (gradient)
H+ADP + Pi ATP
NADPHH+NADP+
H+
Thylakoidcompartment
Chloroplaststroma
Thylakoidmembrane
(a) Light dependent process
(b) Light independent process
CO2
RuBP
Rubisco
Unstable β-ketoacid3-PGA 13-diPGA
ATP
ADP3-PGAL
NADPHH+
NADP+
(Calvin cycle)
trioses tetroses pentoses hexoses (eg glucose) heptoses
Chloroplaststroma
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
CH2OPO32ndash CH2OPO3
2ndash CH2OPO32ndash
CH2OPO32ndash
OOH
HO
HOO
HO
HO2CCO2H
HO
CO2PO32ndash
HO
CHOHO
endash
Cytosol
SCHEME 11 The photosynthetic machinery (PS‐I and PS‐II photosystems I and II)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 3
chloroplast mEP is a starting block in the biosynthesis of terpenes through C
5 isoprene units (isopentenyl diphosphate
(IPP) and dimethylallyl diphosphate (DmAPP)) especially those in C
10 C
20 and C
40 terpenes 3‐PGA is a precursor of
serine and other amino acids while phosphoenolpyruvate (PEP) the precursor of PA is also an intermediate toward the previously mentioned shikimic acid Lastly PA is not only a precursor of the fundamental ldquoC
2rdquo acetyl coenzyme A
(AcCoA) unit but also an intermediate toward aliphatic amino acids and mEP
AcCoA is the building block of fatty acids polyketides and mevalonic acid (mVA) a cytosolic precursor of the C
5 isoprene units for the biosynthesis of terpenes in the
C15
and C30
series (mind it is different from the mEP pathway in product and in cell location) Finally AcCoA enters the citric acid or Krebs cycle which leads to several precursors of amino acids These are oxaloacetic acid precursor of aspartic acid through transamination (thus toward lysine as a nitrogenated C
5N linear unit and methishy
onine as a methyl supplier) and 2‐oxoglutaric acid preshycursor of glutamic acid (and subsequent derivatives such as ornithine as a nitrogenated C
4N linear unit) All these
amino acids are key precursors in the biosynthesis of many alkaloids
114 Reactions Involved in the Construction of Secondary Metabolites
most reactions occurring in the living cells are performed by specialized enzymes which have been classified in an intershynational nomenclature defined by an enzyme commission (EC) number There are six classes of enzymes depending on the biochemical reaction they catalyze EC‐1 oxidoreducshytases (catalyzing oxidoreduction reactions) EC‐2 transfershyases (catalyzing the transfer of functional groups) EC‐3 hydrolases (catalyzing hydrolysis) EC‐4 lyases (breaking bonds through another process than hydrolysis or oxidation leading to a new double bond or a new cycle) EC‐5 isomershyases (catalyzing the isomerization of a molecule) and EC‐6 ligases (forming a covalent bond between two molecules) many subclasses of these enzymes have been described depending on the type of atoms and functional groups involved in the reaction and if any on the cofactor used in this reaction For example several cofactors can be used by dehydrogenases like NAD(P)NAD(P)H FADFADH
2 or
FmNFmNH2 For a description of this classification the
reader can refer to specialized Internet websites like ExplorEnz [1] What is important to realize is that most enzymes are substrate specific and have been selected during
CHOHO
CO2H
O
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
HOOH
HO
HOH2CO
OHC
CH2OPO32ndash
OHHO
CO2H
HO
OH
OH
TYR
PHE
CO2H
NH2
TRP
CH2OPO32ndash
HO
HOH2COH
MEP
C10 C20 and C40 terpenes
CO2HHO
SER
GLY CYS
CO2HOPO3
2ndash
CH2OH
OHHO2C
MVA
C15 and C30 terpenes
Polyketides
Krebscycle
(citric acid)
VAL ALA ILE LEU
CO2H
O
HO2C
CO2HO
CO2H
ASP
LYS
GLU
Glucose-6-phosphate
Fructose-6-phosphate
Erythrose-4-phosphate
3-PGAL
OP2O63ndash OP2O6
3ndash
IPP DMAPP
3-PGA PEP PA
O SCoAAcCoA
3x
Cytosol
Cytosol
Chloroplasts
Chloroplasts
Glycolysis
C2
C5
Shikimic acid Oxaloaceticacid
2-Oxoglutaricacid
MET
THR
PRO
ARGORN
Alkaloids Alkaloids Alkaloids Alkaloids
Aminoacids
Peptidesproteins
Phenolics
C1
C5N C4N
ldquoCH3rdquo
(Indole)C2NC6C3C6C2N C6C1
Pentosephosphatepathway
SCHEME 12 The building block chart involving glycolysis and the Krebs cycle
4 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
evolution to perform specific transformations making natural products with often and yet unknown functions
Secondary metabolites arise from specific biosynthetic pathways which use the previously defined building blocks The bunch of organic reactions involved in these biosynshytheses allows the construction of natural product frameshyworks which are finally diversified through ldquodecorationrdquo steps (Scheme 13) It is not the purpose of this introductive chapter to describe in detail all biosynthetic pathways and the reader can refer to excellent books and articles which have been published elsewhere [2 3]
The reactions involved in the construction of natural product skeletons will be described later for representative classes of compounds The identification of the building block footprint in the natural product skeleton will be emphasized as much as possible sometimes referring to biogenetic speculations [4] After the framework construction the decoration steps will involve as diverse reactions as aliphatic CH oxidations (eg involving a cytochrome P
450 oxygenase) occasionally triggering a
rearrangement heteroatom alkylations (eg methylation by S‐adenosylmethionine) or allylation (by DmAPP) esterifications heteroatom or C‐glycosylations (leading to heterosides) radical couplings (especially for phenols) alcohol oxidations or ketone reductions amineketone transaminations alkene dihydroxylations or epoxidations oxidative halogenations BaeyerndashVilliger oxidations and further oxygenation steps At the end of the biosynthesis such transformations may totally hide the primary building block origin of natural products
115 Secondary Metabolisms
1151 Polyketides Polyketides (or polyacetates) are issued from the oligomerization of C
2 acetate units performed
by polyketide synthases (PKS) and leading to (C2)
n linear
intermediates [5 6] If the (C2)
n intermediates arise from
successive Claisen reactions performed by ketosynthase
domains (KS in nonreducing PKS) a highly reactive poly‐ β‐ketoacyl intermediate H(CH
2CO)
nOH is formed
leading to phenolic and aromatic products through further intramolecular Claisen condensations Furthermore highly reducing PKSs are made of specialized enzymatic subunits working in line or iteratively to functionalize each C
2 linker
bond as CH(OH)CH2 (by ketoreductases (KR)) then as
HCCH (by dehydratases (DH)) and as CH2CH
2 (by enoyl
reductases (ER)) leading to a high degree of functionalization of the final product (Fig 12) By these iterative sequences highly reduced polyketides which can be either linear macrocyclized or polycyclized depending on the reactivity of the biosynthetic intermediates can be formed [7] With the same logic fatty acids are also biosynthesized by fatty acid synthases
moreover the PKS enzyme can be hybridized with nonshyribosomal peptide synthetase (NRPS) domains (see also ldquoNRPS metabolites and peptidesrdquo in the ldquoAlkaloidsrdquo secshytion) leading to the acylation of an amino acid by the (C
2)
n
acyl intermediate As previously the functionalization of the acyl chain depends on the PKS enzyme and the PKSNRPS products are also extremely diversified (eg hirsutellone B Fig 12) [8]
1152 Terpenes Terpenes are derived from the oligoshymerization of the C
5 isoprene units DmAPP and IPP Both
precursors are prompt to generate either an allylic cation (the diphosphate is a good leaving group) or a tertiary carbocation respectively which makes the IPP easy to react with DmAPP (Scheme 14) This reaction happens in the active site of a terpene synthase which activates the deparshyture of the diphosphate group from DmAPP thanks to Lewis acid activation (a metal like mg2+ mn2+ or Co2+ is present in the enzyme active site [9]) This leads to geranyl (C
10
monoterpene precursor) or farnesyl (C15
sesquiterpene precursor) diphosphate depending on the location of the enzyme (chloroplast for the mEP pathway or cytosol for the mVA pathway) Geranylgeranyl (C
20 diterpene) and
Constructionreactions
Decoration reaction(functionalization)Building
blocksNatural product
frameworksNatural products
HH
OH OAcH
HO O OH
OHO
OBz
P450
P450
P450
P450
P450 P450 Oxidativeauxiliaryenzymes
Taxadiene
OP2O63ndash
OP2O63ndash
3times
(a)
(b)
10-Deacetylbaccatin III
DMAPP
IPPand electrophilic
cyclizations
SCHEME 13 (a) From building blocks to natural products and (b) the example of 10‐deacetylbaccatin III
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 5
farnesylfarnesyl (C30
triterpene precursor) diphosphates can also be obtained by further additions of IPP leading to longer linear intermediates
The cyclization of linear precursors is achieved by speshycialized cyclases which generate a poorly functionalized natural product framework [10 11] Auxiliary enzymes such as oxygenases then increase the complexity and the diversity
of compounds by further functionalization (Scheme 13b) [12] A high degree of oxidation can be observed in comshypounds like thapsigargin paclitaxel or bilobalide (Fig 13) The biosynthesis of this last compound for example involves a high oxygenation pattern two Wagnerndashmeerwein rearrangements and several oxidative cleavages leading to the loss of five carbons The resulting natural products can
KS
S-ACP
O O
KR
S-ACP
OH O
S-MAT
O
YesNo
R
R
R
YesNoDH
S-ACP
OR
YesNoER
S-ACP
OR
YesNoTE
OH
OR
New cycle
New cycleor
release
New cycleor
release
New cycleor
release
Release
R in
crem
ente
d by
two
carb
ons
HO
O
S-ACP
O
Claisen condensation (ndashCO2)
reduction (NADPH)
dehydration (ndashH2O)
+
reduction (NADPH)
hydrolysis (H2O)
O O NH
OH
OH
H H
H H
Hirsutellone B(mixed PKSNRPS
product)
OO
O
OHOH
HO OH
Norsolorinic acid
O
O
O
O CHO
OH
HH
H
H
HH
OHHemibrevetoxin B
OOHO
OHO
HOHO OH
Erythronolide A
OH
OH
HO
Panaxytriol
From ahexanoylstartingblock
H
H
Fromtyrosine
H
1C lost fromdecarboxylation
FIGURE 12 Chemical logic of polyketide construction leading to variable functionalization of the elongated acyl chain and examples of resulting chemical diversity ACP acyl carrier protein DH dehydratase ER enoyl reductase KR ketoreductase KS ketosynthase mAT malonyl acyl transferase TE thioesterase
DMAPPOP2O6
3ndashOP2O6
3ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
Geranyl diphosphate (GPP)2 times IPP
H H
Geranylgeranyl diphosphate (GGPP)
Examplesverticillane
casbanetaxane
phorbol
Exampleslabdane
pimaranekauraneabietane
aphidicolanegibberellane
IPP
SCHEME 14 Early assembly of C5 units in terpene biosynthesis leading to diterpenes (C
20)
6 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
thus be extremely modified with structures whose biogenetic origin is far from being obvious at first sight and cannot be determined without further experiments such as isotopic labeling
1153 Flavonoids Resveratrols Gallic Acids and Further Polyphenolics We have previously discussed the polyketide origin of some phenolic compounds based on the (C
2)
n motif Other polyphenols like gallic acids are directly
derived by the aromatization of shikimic acid (C6C
1 building
block Scheme 12) [13] The C6C
3 building blocks are availshy
able from the conversion of phenylalanine and tyrosine into cinnamic and p‐coumaric acids respectively and then by further hydroxylation steps (Scheme 15) These can dimerize into lignans (eg podophyllotoxin) [14 15] through radical processes or converted to low molecular weight compounds like eugenol coumarins or vanillin [16] The coenzyme A thioesters of these C
6C
3 acids can be used as initiator units
by specialized ketosynthases for an elongation by two acetyl units leading to aromatic polyketides like styrylpyrones or diarylheptanoids (eg curcumin) [17] Important comshypounds from this metabolism are flavonoids (C
6C
3C
6) [18]
and stilbenoids (C6C
2C
6) (a decarboxylation occurs during
the aryl cyclization) [19] which are synthesized by chalcone synthase and stilbene synthase respectively Flavonoids (eg catechin) and stilbenes (eg resveratrol) are present in large amounts in fruits and vegetables and may exert their radical scavenging properties in vivo
1154 Alkaloids Alkaloids are nitrogen‐containing compounds The nitrogen(s) can be involved in an amine function conferring basicity to the natural product (like ldquoalkalirdquo) or in less or nonbasic functions such as an amide a nitrile an isonitrile or an ammonium salt (quaternary amines) For amines protonation often occurs at physiological pH and may condition their biological activity In many cases the nitrogen is biogenetically derived from an amino acid We will thus discuss alkaloids according to their amino acid origin
Alkaloids Derived from the Krebs Cycle (Lysine and Ornithine Derived) As shown previously (Scheme 12) the Krebs cycle is a crucial metabolic process which leads to α‐ketoacids (oxaloacetic and 2‐oxoglutaric acids) Their enzymatic transamination affords the two amino acidsmdashaspartic acid and glutamic acidmdashwhich are the direct biosynthetic precursors of amino acids lysine and ornithine respectively These in turn produce cadaverine a ldquoC
5Nrdquo unit
and putrescine a ldquoC4Nrdquo unit which are major components
for the biosynthesis of important alkaloids as will be discussed later (Scheme 16) Additionally ornithine is a precursor of arginine another important amino acid
ornithine‐derived alkaloids (incorporating the c4n
unit) Putrescine is derived from the decarboxylation of ornithine and is a precursor of linear polyamines like spermshyine After enzymatic methylation of one amine of putrescine
C10 C15
C15
C10
C10
C30
CO2H
Chrysanthemic acid
OH
Menthol
O
HO
H
HOGlc
MeO2CLoganin (iridoid)
H
H
H
HH
OO
OParthenolide
O
O
OHOHO
OAcHO
OnC3H7
O
O
O
nC7H15
Thapsigargin
O
O
OO
H
H
O
Cleavage
H
H
Artemisinin
C20 C40
O
O
OO
OO
OHOHH
Bilobalide(highly rearrangedand missing 5 C)
CleavageH
Highoxidativecleavages
HO OAcH
AcO O OH
OOOBz
O
Ph
BzNH
OHFrom PHE Paclitaxel
C25O
H
HO
OHCH
OH Ophiobolin A
H
HO
H
H
HHCholesterol
(missing 3 C WM)
H
H H H
H
H
Hopene
β-Carotene
C30 - 3
C15
C20 - 5WM
WM
FIGURE 13 Chemical diversity in the terpene series (Wm Wagnerndashmeerwein shifts lost carbons bold bonds are remnant of primary building blocks)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 7
in the presence of S‐adenosylmethionine transamination of the other affords γ‐(N‐methylamino)aldehyde [20] The resulting cyclic iminium is a key intermediate in the formation of many medicinally important alkaloids such as
the plant‐derived compounds cocaine atropine or the calysshytegines [21 22] Indeed this iminium is a mannich acceptor which can react with various nucleophiles the first of those being the carbanion of acetyl‐CoA Thus after a stepwise
CO2H
RCinnamic acid (R=H)p-Coumaric acid (R=OH)
RO
PHETYR
OH
[H] [O]
lignans
OH
O
O
O
O
OMeMeO
OMe
C mdash C andor C mdash Oradical couplings
After E Zisomerization
for example Podophyllotoxin
O ORO
Coumarins(eg scopoletin)
[O]
Prenylationcyclizationcleavage[O]
O OO
Furocoumarins(eg psoralen)
RO
nAcetyl-CoA
thencyclization
n = 2 styrylpyrones
O O
OMe
n = 3 avonoids(from chalcone synthase)
C6C3
H
H H
From AcCoA
O
n = 3 stilbenoids(from stilbene synthase)
HO
OH
OHFrom AcCoA From AcCoA(ndashCO2)
Resveratrol
HO
OH
OH
OH
OH
H
Catechin
MeO Yangonin
From AcCoA
SCHEME 15 The phenylpropanoid biosynthetic pathways
LYS
CO2
Cadaverine
O NH
H2O
NH
Piperideineiminium
Pelletierine
AcAcCoA
CO2
NH
O
N
O
Pseudopelletierine
NH
Ph
OH
Ph
O
Lobeline
NH
δ+
δndash
N
H
H
N
H
OH
Lupinine
N
H N
H
(+)-Sparteine
N
NH
O (ndash)-Cytisine
NH
CO2H
Pipecolic acid
N
OHHO
OH
HO
H
Castanos permine
ORN
CO2
NH2 NH2 NH2 NH2
NH2
NH2
NH2
PutrescineO
NHMe
N-Methylpyrrolinium
2 timesAcCoA
NMe
ARG
n = 1 spermidinen = 2 homospermidine
NMe
O
HygrineCO2
CO2
MeN
O
[O]
TropinoneCO2
HNHO
OHOH
OH
Calystegine B2
MeN
OAtropine
O
Ph
HO
NMe
NMe
NMe
O
Cuscohygrine
HN
HN
NH2
( )n
O
n = 2
N
HHO
Retronecine
HH
H
H
H
HO
H H
H
SCHEME 16 Lysine‐ and ornithine‐derived alkaloid biosynthetic pathways (mind the structural similarities)
8 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
elongation by two AcCoA units either decarboxylation can occur leading to the acetonylpyrrolidine hygrine or a secshyond mannich reaction by the intramolecular attack of the acetoacetate anion onto an oxidation‐derived pyrrolinium leading to the tropane skeleton (tropinone) The acetoacetyl‐CoA intermediate can also react intermolecularly with another pyrrolinium cation leading to cuscohygrine after decarboxylation Finally the pyrrolizidine alkaloids [23] are derived from homospermidine which when submitted to terminal oxidative deamination leads to the bicyclic skelshyeton of retronecine and further Senecio alkaloids We can mention herein that ornithine is a biosynthetic precursor of arginine bearing a guanidine function which is an intermediate toward the toxic compounds tetrodotoxin and saxitoxin (not shown)
lysine‐derived alkaloids (incorporating the c5n
unit) From lysine to piperidine alkaloids the biosynthetic steps parallel the one previously described from ornithine Indeed the oxidative deamination of cadaverine affords a δ‐amino aldehyde which cyclizes through imine formation into piperideine Protonation results in a mannich acceptor which is able to react with various nucleophiles such as β‐ketothioester anions The first product of these reactions is pelletierine which can further react through an intramolecular mannich
reaction leading to pseudopelletierine Quinolizidines [24] can also be formed first from the mannich reaction of the piperideine acceptor with the corresponding enamine nucleoshyphile and then after additional transformation steps leading for example to lupinine sparteine or cytisine
Indolizidine alkaloids [15] such as castanospermine and swainsonine are formed from pipecolic acid an amino acid derived from lysine which can be elongated by malonyl‐CoA followed by ring closure When protonated these alkaloids are oxonium mimics strongly inhibiting glycosidases
Tyrosine‐ and Phenylalanine‐Derived Alkaloids Tyrosine and phenylalanine amino acids are bearing the phenylshyethylamine moiety of many medicinally relevant alkaloids Further hydroxylations on the aromatic carbocycle or on the aliphatic part can be observed methylations can occur on phenolic oxygens and on the amine leading to catecholamines (adrenaline noradrenaline dopamine) Arylethylamines are also usual to react with endogenous aldehydes through PictetndashSpengler reactions [25] leading to important biosynthetic intermediates (Scheme 17) like
bull Reticuline from the reaction with 4‐hydroxyphenylacetshyaldehyde toward benzyltetrahydroisoquinoline alkaloids
NH2
Phenylethylamines
RONH TetrahydroisoquinolinesRO
RʹCHO
Rʹ
NH
(CH2)n
MeO
HO
Benzyltetrahydroisoquinoline(n = 1) for example reticuline
orPhenethyltetrahydroisoquinoline
(n = 2) eg autumnaline
IntermolecularCmdashO phenol
couplings
Curarealkaloids
IntramolecularC mdash C phenol
couplings
ONMe
HHO
H
HO
Morphine
NMe
MeO
HO
MeOOH
Isoboldine
O
O
OMe
NO2
CO2H
Aristolochic acid
OMe
O
NHAcMeO
MeOMeO
Colchicine
N
O
O
OMe
OMeBerberine
IntramolecularCmdash C coupling
and furtheroxidative events
Rʹ derivedfrom PHE
Pictet-Spenglerreaction
TYR
HO
HO CHO
HNHO
HO
OH
OMeO
OH
NMe
[O]
GalantamineNorbelladine
Rʹ derived fromsecologanin
NAc
HO
HO
O
CO2MeH
H
H
OHIpecosideaglycone
Ipecac alkaloids
1ClostCmdash C cleavage
and ring extension
H
ROH
1C lost
Cleavage
SCHEME 17 Tyrosine‐derived alkaloid biosynthetic pathways (double head arrows figure bond cleavages during biosynthetic processes)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 9
morphine berberine tubocurarine isoboldine or the highly modified aristolochic acid [26 27]
bull Automnaline from the reaction with 3‐(4‐hydroxyphenyl)propanal toward phenylethyltetrahydroisoquinoline alkaloids colchicine cephalotaxine or schelhammerishycine [28 29]
bull Ipecoside from the reaction of dopamine with secoloshyganin toward terpene tetrahydroisoquinoline alkaloids ipecoside or emetine
Lastly norbelladine (top of Scheme 17) is issued from the reductive amination of 34‐dihydroxybenzaldehyde (derived from phenylalanine) with tyramine (derived from tyrosine) and constitutes a biosynthetic node leading to Amaryllidaceae alkaloids such as galantamine crinine or lycorine depending on the topology of phenolic couplings In all these biosynthetic routes radical phenolic couplings are key reactions for CC and CO bond formations and rearrangements [30 31]
Tryptophan‐Derived Indole and Indole Monoterpene Alkaloids As for alkaloids derived from tyrosine and phenylalanine those derived from tryptophan are formed after decarboxylation of the amino acid (into tryptamine) and possible hydroxylation of the aromatic carbocycle (eg serotonin) and N‐methylation (eg psilocin) As previously tryptamine can also react through PictetndashSpengler reactions to form tetrahydro‐β‐carbolines which can be aromatized for example into harmine (Scheme 18) [16]
When the aldehyde partner of the PictetndashSpengler reacshytion with tryptamine is the terpene secologanin strictosidine is formed as an entry toward the vast monoterpene indole alkaloids [32 33] Hydrolysis of the glucosidic part releases the strictosidine aglycone bearing an aldehyde while iminshyium formation and further cyclization and reduction can lead to ajmalicine (from oxocyclization) or yohimbine (from carshybocyclization) These alkaloids are referred to as from the Corynanthe type with the monoterpene carbon skeleton unmodified Although it misses one carbon and has a very
RO
Carbonylcompounds
TRPNH
NH2
Indolethylamines(eg serotonine)
RONH
NH
Rʹ NH
N
MeOPictet-Spenglerreaction for example Harmineβ-Carbolines
NH
NHH
O
MeO2C
MeO2C
OH
H
H
Rʹ derived from secologanin
Strictosidine aglycone
NH
N
OH
H
H
Ajmalicine
N
N
O
O
H
H
H
Strychnine (C lost)Corynanthe type
Aspidosperma type Iboga type
N
N
OH
OMe
Quinine (C lost)
N
NH CO2Me
Catharanthine
NH
N
CO2MeH
OAc
OH
Vindoline
Complextransformations
NH
NMe
HN
MeN
H
H
Chimonanthine
Cyclizationdimerization
Prenylationcyclization
NH
NMeHO2C
H
Lysergic acid
H
H
Ergot alkaloidsPyrroloindoles Indole monoterpene
1C lost
1C lostFrom AcCoA
SCHEME 18 Tryptophan‐derived alkaloid biosynthetic pathways (gray parts monoterpenic units)
10 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
different structure strychnine is related to the Corynanthe alkaloids incorporating two carbons from acetyl‐CoA Highly modified monoterpene skeletons are derived from the Corynanthe core through CC bond breaking and reorshyganization leading to Iboga‐type (eg catharanthine) and Aspidosperma‐type (eg vindoline) alkaloids The antishycancer drug vinblastine is a heterodimer resulting from the nucleophilic attack of vindoline on a mannich acceptor resulting from catharanthine found in madagascar perishywinkle (Catharanthus roseus) The heteroaromatic comshypounds ellipticine camptothecin and quinine are also derived from a Corynanthe‐type precursor although in this case the biosynthetic relationship may not be obvious due to deep modifications of the skeleton
Finally two important classes of compounds have to be mentioned since they have inspired many synthetic chemists The pyrroloindole alkaloids result from the cyclization of tryptamine as found in physostigmine (formed by a cationic mechanism after methylation in position 3 of the indole not shown) or in chimonanthine (presumably formed by a radical coupling mechanism Scheme 18) The ergot alkaloids are derived from the 33‐dimethylallylation on position 4 of the indole in trypshytophan whose further cyclization and oxidation processes afford the natural products (eg lysergic acid Scheme 18 and ergotamine) which have had important medical applications [34]
NRPS Metabolites and Peptides NRPS enzymes assemble amino acids including nonproteinogenic ones into oligopeptides The enzymes contain several modules and especially an adenylation domain (A) which specifically selects and activates the amino acid to be transferred as a thioester on the nearby peptidyl carrier protein (PCP) [2] A condensation module (C) then catalyzes the formation of the peptide bonds between the newly introduced amino acyl‐PCP (bearing a free amine) and the elongated peptidyl‐PCP thioester At the end of the elongation a cyclization can occur into cyclopeptides but the peptide can also be
transferred to auxiliary enzymes like methyltransferases glycosyltransferases or oxidases (vancomycins are typical products of such functionalizations) [35 36] The formation of heterocycles is also frequently encountered in this metabolism as in penicillins that are derived from the tripeptide α‐aminoadipoyl‐cysteinyl‐valine or telomestatin (Fig 14) [2]
Other Alkaloid Origins There are many other nitrogen sources involved in alkaloid biosyntheses for example nicotinic acid (originated from aspartic acid and intershymediate in nicotine and anabasine biosyntheses) and anthranilic acid (originated from tryptophan and intermedishyate toward acridines or aurachins) The amination reaction (eg through transamination of carbonyl compounds) is also a way to introduce nitrogens in natural products for example from fatty acids steroids (toward Solanum alkaloids or cyclopamines) or other terpenoids (aconitine and atisine have diterpene skeletons while Daphniphyllum alkaloids are triterpene derivatives) Finally nucleic acids can also be precursors of alkaloids like the well‐known caffeine
12 FROM BIOSYNTHESIS TO TOTAL SYNTHESIS STRATEGIES TOWARD THE NATURAL PRODUCT CHEMICAL SPACE
121 The Chemical Space of Natural Products
Natural products occupy an important place in human com munities as demonstrated by their vast use from ancient times to nowadays like dyes fibers oils pershyfumes agrochemicals or drugs Broadly both primary and secondary metabolites could be classified as natural products while the latter as discussed previously are usushyally regarded as the ldquonatural productsrdquo owing to their comshyplexity and diversity arising from a variety of biosynthetic pathways The structural chemical diversity found among
N
S
CO2HO
HNPhH2C
O
Penicillin G
OH
HN
HN
O
O
ONH2
O
OHO
NHMe
OCl
O
NH
NH
NH
O
ClHO
O
HN
H
H
HOOH
OHHO2C
Vancomycin aglycone
ON
NO
O
NN
O
O
N
N O
Me
S
N N
OMeH
Telomestatin
L-AAA-L-CYS-D-VAL
Enzymes
FIGURE 14 Structural diversity of nonribosomal peptide compounds (AAA α‐aminoadipic acid)
FROm BIOSyNTHESIS TO TOTAL SyNTHESIS STRATEGIES TOWARD THE NATuRAL PRODuCT CHEmICAL SPACE 11
all living organisms defining the chemical space of natural products [37] is the consequence of their evolution occurshyring as an adaptation of organisms to their environment It is commonly believed that secondary metabolites are proshyduced as messengers by living organisms or as weapons against enemies and thus they should have certain biological activities in a medicinal point of view [38] Indeed natural products are regarded as one of the main sources of medicines (Fig 15) From the traditional medicinal extracts to every single bioactive molecule the methods of extraction purification identification and biological investigation of natural products have been well established Their complex structures and interesting properties have attracted synthetic chemists to accomplish their total syntheses and that of medicinally relevant anashylogs sometimes in the industrial context [39] Thus tarshygeting the chemical space of natural products has never been more relevant than today Although the discovery of natural products demands time and labor‐consuming manipulations it is worth to notice that the knowledge on this chemical space is still continually growing while biological advances allow for discovering and undershystanding potential targets However increasing the chemical space of human‐made compounds based on natural products should benefit from transdisciplinary colshylaborations such as the use of coupled biosynthetic and chemical synthetic methods to design original ldquounnatural natural productsrdquo [40]
122 The Biosynthetic Pathways as an Inspiration for Synthetic Challenges
1221 Precursor‐Directed Biosyntheses and Mutashysynthetic Strategies to Increase the Chemical Space of Natural Products As the genetics and biochemistry of natural product biosynthesis are better understood novel biosynthetic techniques have been developed to study and generate new diversity in natural product analogs (Scheme 19) Precursor‐directed biosynthesis (PDB) is considered as the earliest example of combining chemical and biological methods for the generation of complex natural product analogs [41 42] This approach compared with the biosynthetic pathway of wild‐type metabolites (Scheme 19a) involves the feeding of analogs of the natural biosynthetic building blocks to the living organisms (Scheme 19b) usually bacteria or fungi which incorporate the modified precursors into the biosynthesized compound mutasynthesis also termed as mutational biosynthesis (mBS) involves the inactivation of a key step of the bioshysynthesis in a mutant microorganism (Scheme 19c) which can then be fed by various modified or advanced building blocks (mutasynthons Scheme 19d) [43] These mutasshyynthons could not be incorporated by the wild type due to specificities of the enzymatic machinery Build up on PDB mBS eliminates the natural biosynthetic intermediate thus generating a less complex mixture of metabolites and making the purification or yield of target products better
O NH
OHO
O
OO
O
O
OH
HOO
O O
OH
HOO
NH ONH2
O
ONH
NH
O
OCl Cl
O
O
OH
HN
HN
HN
HN
OO
HO
HN
OH
OHOH
O
O
HO
HO
OHO
O
OHH2N
O
OH3C
H
O
H
CH3
CH3
H
O O
NO
H
O
HO
O
NO
O
NH
HN
OHO
HONH
ON
H
HO
O
H2N
O OH
ONH
OH
ON OHO
OH
HO OS OH
OOOH
OHO
O
ON
OO
O
HO
O
O OH
OO
Micafunginantifungal
Rapamycinimmunosuppressive
Galantamineanti-Alzheimer
Taxolanticancer
Artemisininantimalarial
Vancomycinantibiotic
FIGURE 15 Some famous natural products currently used as drugs
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product
2 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
(Scheme 12) Important intermediates for the secondary metabolism are produced during glycolysis Glucose glucose‐6‐phosphate and fructose‐6‐phosphate can be converted to other hexoses and pentoses that can be oligoshymerized and enter in the composition of heterosides Additionally fructose‐6‐phosphate connects the pentose
phosphate pathway leading to erythrose‐4‐phosphate toward shikimic acid which is a key metabolite in the biosynthesis of aromatic amino acids (phenylalanine tyrosine or C
6C
3
units) and C6C
1 phenolic compounds The next important
intermediate in glycolysis is 3‐PGAL which can be redishyrected toward methylerythritol‐4‐phosphate (mEP) in the
Primary metabolism Secondary metabolism
The field ofbiochemistry
The field ofnatural product
chemistry
Essential toliving organisms
Essential to theproducer organisms
under particularconditions
Nucleic acids (DNARNA) carbohydrates
lipids amino acidspeptides and proteins
Alkaloids terpenespolyketides
polyphenols andtheir heterosidic form
Biological effects(defense signaling)
Biosyntheticpathways
Main compoundclasses
FIGURE 11 Primary versus secondary metabolisms
PS-I
PS-IIendash
hν H2O H+ and O2
ATP synthase
NADP reductase
H+ (gradient)
H+ADP + Pi ATP
NADPHH+NADP+
H+
Thylakoidcompartment
Chloroplaststroma
Thylakoidmembrane
(a) Light dependent process
(b) Light independent process
CO2
RuBP
Rubisco
Unstable β-ketoacid3-PGA 13-diPGA
ATP
ADP3-PGAL
NADPHH+
NADP+
(Calvin cycle)
trioses tetroses pentoses hexoses (eg glucose) heptoses
Chloroplaststroma
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
CH2OPO32ndash CH2OPO3
2ndash CH2OPO32ndash
CH2OPO32ndash
OOH
HO
HOO
HO
HO2CCO2H
HO
CO2PO32ndash
HO
CHOHO
endash
Cytosol
SCHEME 11 The photosynthetic machinery (PS‐I and PS‐II photosystems I and II)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 3
chloroplast mEP is a starting block in the biosynthesis of terpenes through C
5 isoprene units (isopentenyl diphosphate
(IPP) and dimethylallyl diphosphate (DmAPP)) especially those in C
10 C
20 and C
40 terpenes 3‐PGA is a precursor of
serine and other amino acids while phosphoenolpyruvate (PEP) the precursor of PA is also an intermediate toward the previously mentioned shikimic acid Lastly PA is not only a precursor of the fundamental ldquoC
2rdquo acetyl coenzyme A
(AcCoA) unit but also an intermediate toward aliphatic amino acids and mEP
AcCoA is the building block of fatty acids polyketides and mevalonic acid (mVA) a cytosolic precursor of the C
5 isoprene units for the biosynthesis of terpenes in the
C15
and C30
series (mind it is different from the mEP pathway in product and in cell location) Finally AcCoA enters the citric acid or Krebs cycle which leads to several precursors of amino acids These are oxaloacetic acid precursor of aspartic acid through transamination (thus toward lysine as a nitrogenated C
5N linear unit and methishy
onine as a methyl supplier) and 2‐oxoglutaric acid preshycursor of glutamic acid (and subsequent derivatives such as ornithine as a nitrogenated C
4N linear unit) All these
amino acids are key precursors in the biosynthesis of many alkaloids
114 Reactions Involved in the Construction of Secondary Metabolites
most reactions occurring in the living cells are performed by specialized enzymes which have been classified in an intershynational nomenclature defined by an enzyme commission (EC) number There are six classes of enzymes depending on the biochemical reaction they catalyze EC‐1 oxidoreducshytases (catalyzing oxidoreduction reactions) EC‐2 transfershyases (catalyzing the transfer of functional groups) EC‐3 hydrolases (catalyzing hydrolysis) EC‐4 lyases (breaking bonds through another process than hydrolysis or oxidation leading to a new double bond or a new cycle) EC‐5 isomershyases (catalyzing the isomerization of a molecule) and EC‐6 ligases (forming a covalent bond between two molecules) many subclasses of these enzymes have been described depending on the type of atoms and functional groups involved in the reaction and if any on the cofactor used in this reaction For example several cofactors can be used by dehydrogenases like NAD(P)NAD(P)H FADFADH
2 or
FmNFmNH2 For a description of this classification the
reader can refer to specialized Internet websites like ExplorEnz [1] What is important to realize is that most enzymes are substrate specific and have been selected during
CHOHO
CO2H
O
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
HOOH
HO
HOH2CO
OHC
CH2OPO32ndash
OHHO
CO2H
HO
OH
OH
TYR
PHE
CO2H
NH2
TRP
CH2OPO32ndash
HO
HOH2COH
MEP
C10 C20 and C40 terpenes
CO2HHO
SER
GLY CYS
CO2HOPO3
2ndash
CH2OH
OHHO2C
MVA
C15 and C30 terpenes
Polyketides
Krebscycle
(citric acid)
VAL ALA ILE LEU
CO2H
O
HO2C
CO2HO
CO2H
ASP
LYS
GLU
Glucose-6-phosphate
Fructose-6-phosphate
Erythrose-4-phosphate
3-PGAL
OP2O63ndash OP2O6
3ndash
IPP DMAPP
3-PGA PEP PA
O SCoAAcCoA
3x
Cytosol
Cytosol
Chloroplasts
Chloroplasts
Glycolysis
C2
C5
Shikimic acid Oxaloaceticacid
2-Oxoglutaricacid
MET
THR
PRO
ARGORN
Alkaloids Alkaloids Alkaloids Alkaloids
Aminoacids
Peptidesproteins
Phenolics
C1
C5N C4N
ldquoCH3rdquo
(Indole)C2NC6C3C6C2N C6C1
Pentosephosphatepathway
SCHEME 12 The building block chart involving glycolysis and the Krebs cycle
4 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
evolution to perform specific transformations making natural products with often and yet unknown functions
Secondary metabolites arise from specific biosynthetic pathways which use the previously defined building blocks The bunch of organic reactions involved in these biosynshytheses allows the construction of natural product frameshyworks which are finally diversified through ldquodecorationrdquo steps (Scheme 13) It is not the purpose of this introductive chapter to describe in detail all biosynthetic pathways and the reader can refer to excellent books and articles which have been published elsewhere [2 3]
The reactions involved in the construction of natural product skeletons will be described later for representative classes of compounds The identification of the building block footprint in the natural product skeleton will be emphasized as much as possible sometimes referring to biogenetic speculations [4] After the framework construction the decoration steps will involve as diverse reactions as aliphatic CH oxidations (eg involving a cytochrome P
450 oxygenase) occasionally triggering a
rearrangement heteroatom alkylations (eg methylation by S‐adenosylmethionine) or allylation (by DmAPP) esterifications heteroatom or C‐glycosylations (leading to heterosides) radical couplings (especially for phenols) alcohol oxidations or ketone reductions amineketone transaminations alkene dihydroxylations or epoxidations oxidative halogenations BaeyerndashVilliger oxidations and further oxygenation steps At the end of the biosynthesis such transformations may totally hide the primary building block origin of natural products
115 Secondary Metabolisms
1151 Polyketides Polyketides (or polyacetates) are issued from the oligomerization of C
2 acetate units performed
by polyketide synthases (PKS) and leading to (C2)
n linear
intermediates [5 6] If the (C2)
n intermediates arise from
successive Claisen reactions performed by ketosynthase
domains (KS in nonreducing PKS) a highly reactive poly‐ β‐ketoacyl intermediate H(CH
2CO)
nOH is formed
leading to phenolic and aromatic products through further intramolecular Claisen condensations Furthermore highly reducing PKSs are made of specialized enzymatic subunits working in line or iteratively to functionalize each C
2 linker
bond as CH(OH)CH2 (by ketoreductases (KR)) then as
HCCH (by dehydratases (DH)) and as CH2CH
2 (by enoyl
reductases (ER)) leading to a high degree of functionalization of the final product (Fig 12) By these iterative sequences highly reduced polyketides which can be either linear macrocyclized or polycyclized depending on the reactivity of the biosynthetic intermediates can be formed [7] With the same logic fatty acids are also biosynthesized by fatty acid synthases
moreover the PKS enzyme can be hybridized with nonshyribosomal peptide synthetase (NRPS) domains (see also ldquoNRPS metabolites and peptidesrdquo in the ldquoAlkaloidsrdquo secshytion) leading to the acylation of an amino acid by the (C
2)
n
acyl intermediate As previously the functionalization of the acyl chain depends on the PKS enzyme and the PKSNRPS products are also extremely diversified (eg hirsutellone B Fig 12) [8]
1152 Terpenes Terpenes are derived from the oligoshymerization of the C
5 isoprene units DmAPP and IPP Both
precursors are prompt to generate either an allylic cation (the diphosphate is a good leaving group) or a tertiary carbocation respectively which makes the IPP easy to react with DmAPP (Scheme 14) This reaction happens in the active site of a terpene synthase which activates the deparshyture of the diphosphate group from DmAPP thanks to Lewis acid activation (a metal like mg2+ mn2+ or Co2+ is present in the enzyme active site [9]) This leads to geranyl (C
10
monoterpene precursor) or farnesyl (C15
sesquiterpene precursor) diphosphate depending on the location of the enzyme (chloroplast for the mEP pathway or cytosol for the mVA pathway) Geranylgeranyl (C
20 diterpene) and
Constructionreactions
Decoration reaction(functionalization)Building
blocksNatural product
frameworksNatural products
HH
OH OAcH
HO O OH
OHO
OBz
P450
P450
P450
P450
P450 P450 Oxidativeauxiliaryenzymes
Taxadiene
OP2O63ndash
OP2O63ndash
3times
(a)
(b)
10-Deacetylbaccatin III
DMAPP
IPPand electrophilic
cyclizations
SCHEME 13 (a) From building blocks to natural products and (b) the example of 10‐deacetylbaccatin III
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 5
farnesylfarnesyl (C30
triterpene precursor) diphosphates can also be obtained by further additions of IPP leading to longer linear intermediates
The cyclization of linear precursors is achieved by speshycialized cyclases which generate a poorly functionalized natural product framework [10 11] Auxiliary enzymes such as oxygenases then increase the complexity and the diversity
of compounds by further functionalization (Scheme 13b) [12] A high degree of oxidation can be observed in comshypounds like thapsigargin paclitaxel or bilobalide (Fig 13) The biosynthesis of this last compound for example involves a high oxygenation pattern two Wagnerndashmeerwein rearrangements and several oxidative cleavages leading to the loss of five carbons The resulting natural products can
KS
S-ACP
O O
KR
S-ACP
OH O
S-MAT
O
YesNo
R
R
R
YesNoDH
S-ACP
OR
YesNoER
S-ACP
OR
YesNoTE
OH
OR
New cycle
New cycleor
release
New cycleor
release
New cycleor
release
Release
R in
crem
ente
d by
two
carb
ons
HO
O
S-ACP
O
Claisen condensation (ndashCO2)
reduction (NADPH)
dehydration (ndashH2O)
+
reduction (NADPH)
hydrolysis (H2O)
O O NH
OH
OH
H H
H H
Hirsutellone B(mixed PKSNRPS
product)
OO
O
OHOH
HO OH
Norsolorinic acid
O
O
O
O CHO
OH
HH
H
H
HH
OHHemibrevetoxin B
OOHO
OHO
HOHO OH
Erythronolide A
OH
OH
HO
Panaxytriol
From ahexanoylstartingblock
H
H
Fromtyrosine
H
1C lost fromdecarboxylation
FIGURE 12 Chemical logic of polyketide construction leading to variable functionalization of the elongated acyl chain and examples of resulting chemical diversity ACP acyl carrier protein DH dehydratase ER enoyl reductase KR ketoreductase KS ketosynthase mAT malonyl acyl transferase TE thioesterase
DMAPPOP2O6
3ndashOP2O6
3ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
Geranyl diphosphate (GPP)2 times IPP
H H
Geranylgeranyl diphosphate (GGPP)
Examplesverticillane
casbanetaxane
phorbol
Exampleslabdane
pimaranekauraneabietane
aphidicolanegibberellane
IPP
SCHEME 14 Early assembly of C5 units in terpene biosynthesis leading to diterpenes (C
20)
6 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
thus be extremely modified with structures whose biogenetic origin is far from being obvious at first sight and cannot be determined without further experiments such as isotopic labeling
1153 Flavonoids Resveratrols Gallic Acids and Further Polyphenolics We have previously discussed the polyketide origin of some phenolic compounds based on the (C
2)
n motif Other polyphenols like gallic acids are directly
derived by the aromatization of shikimic acid (C6C
1 building
block Scheme 12) [13] The C6C
3 building blocks are availshy
able from the conversion of phenylalanine and tyrosine into cinnamic and p‐coumaric acids respectively and then by further hydroxylation steps (Scheme 15) These can dimerize into lignans (eg podophyllotoxin) [14 15] through radical processes or converted to low molecular weight compounds like eugenol coumarins or vanillin [16] The coenzyme A thioesters of these C
6C
3 acids can be used as initiator units
by specialized ketosynthases for an elongation by two acetyl units leading to aromatic polyketides like styrylpyrones or diarylheptanoids (eg curcumin) [17] Important comshypounds from this metabolism are flavonoids (C
6C
3C
6) [18]
and stilbenoids (C6C
2C
6) (a decarboxylation occurs during
the aryl cyclization) [19] which are synthesized by chalcone synthase and stilbene synthase respectively Flavonoids (eg catechin) and stilbenes (eg resveratrol) are present in large amounts in fruits and vegetables and may exert their radical scavenging properties in vivo
1154 Alkaloids Alkaloids are nitrogen‐containing compounds The nitrogen(s) can be involved in an amine function conferring basicity to the natural product (like ldquoalkalirdquo) or in less or nonbasic functions such as an amide a nitrile an isonitrile or an ammonium salt (quaternary amines) For amines protonation often occurs at physiological pH and may condition their biological activity In many cases the nitrogen is biogenetically derived from an amino acid We will thus discuss alkaloids according to their amino acid origin
Alkaloids Derived from the Krebs Cycle (Lysine and Ornithine Derived) As shown previously (Scheme 12) the Krebs cycle is a crucial metabolic process which leads to α‐ketoacids (oxaloacetic and 2‐oxoglutaric acids) Their enzymatic transamination affords the two amino acidsmdashaspartic acid and glutamic acidmdashwhich are the direct biosynthetic precursors of amino acids lysine and ornithine respectively These in turn produce cadaverine a ldquoC
5Nrdquo unit
and putrescine a ldquoC4Nrdquo unit which are major components
for the biosynthesis of important alkaloids as will be discussed later (Scheme 16) Additionally ornithine is a precursor of arginine another important amino acid
ornithine‐derived alkaloids (incorporating the c4n
unit) Putrescine is derived from the decarboxylation of ornithine and is a precursor of linear polyamines like spermshyine After enzymatic methylation of one amine of putrescine
C10 C15
C15
C10
C10
C30
CO2H
Chrysanthemic acid
OH
Menthol
O
HO
H
HOGlc
MeO2CLoganin (iridoid)
H
H
H
HH
OO
OParthenolide
O
O
OHOHO
OAcHO
OnC3H7
O
O
O
nC7H15
Thapsigargin
O
O
OO
H
H
O
Cleavage
H
H
Artemisinin
C20 C40
O
O
OO
OO
OHOHH
Bilobalide(highly rearrangedand missing 5 C)
CleavageH
Highoxidativecleavages
HO OAcH
AcO O OH
OOOBz
O
Ph
BzNH
OHFrom PHE Paclitaxel
C25O
H
HO
OHCH
OH Ophiobolin A
H
HO
H
H
HHCholesterol
(missing 3 C WM)
H
H H H
H
H
Hopene
β-Carotene
C30 - 3
C15
C20 - 5WM
WM
FIGURE 13 Chemical diversity in the terpene series (Wm Wagnerndashmeerwein shifts lost carbons bold bonds are remnant of primary building blocks)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 7
in the presence of S‐adenosylmethionine transamination of the other affords γ‐(N‐methylamino)aldehyde [20] The resulting cyclic iminium is a key intermediate in the formation of many medicinally important alkaloids such as
the plant‐derived compounds cocaine atropine or the calysshytegines [21 22] Indeed this iminium is a mannich acceptor which can react with various nucleophiles the first of those being the carbanion of acetyl‐CoA Thus after a stepwise
CO2H
RCinnamic acid (R=H)p-Coumaric acid (R=OH)
RO
PHETYR
OH
[H] [O]
lignans
OH
O
O
O
O
OMeMeO
OMe
C mdash C andor C mdash Oradical couplings
After E Zisomerization
for example Podophyllotoxin
O ORO
Coumarins(eg scopoletin)
[O]
Prenylationcyclizationcleavage[O]
O OO
Furocoumarins(eg psoralen)
RO
nAcetyl-CoA
thencyclization
n = 2 styrylpyrones
O O
OMe
n = 3 avonoids(from chalcone synthase)
C6C3
H
H H
From AcCoA
O
n = 3 stilbenoids(from stilbene synthase)
HO
OH
OHFrom AcCoA From AcCoA(ndashCO2)
Resveratrol
HO
OH
OH
OH
OH
H
Catechin
MeO Yangonin
From AcCoA
SCHEME 15 The phenylpropanoid biosynthetic pathways
LYS
CO2
Cadaverine
O NH
H2O
NH
Piperideineiminium
Pelletierine
AcAcCoA
CO2
NH
O
N
O
Pseudopelletierine
NH
Ph
OH
Ph
O
Lobeline
NH
δ+
δndash
N
H
H
N
H
OH
Lupinine
N
H N
H
(+)-Sparteine
N
NH
O (ndash)-Cytisine
NH
CO2H
Pipecolic acid
N
OHHO
OH
HO
H
Castanos permine
ORN
CO2
NH2 NH2 NH2 NH2
NH2
NH2
NH2
PutrescineO
NHMe
N-Methylpyrrolinium
2 timesAcCoA
NMe
ARG
n = 1 spermidinen = 2 homospermidine
NMe
O
HygrineCO2
CO2
MeN
O
[O]
TropinoneCO2
HNHO
OHOH
OH
Calystegine B2
MeN
OAtropine
O
Ph
HO
NMe
NMe
NMe
O
Cuscohygrine
HN
HN
NH2
( )n
O
n = 2
N
HHO
Retronecine
HH
H
H
H
HO
H H
H
SCHEME 16 Lysine‐ and ornithine‐derived alkaloid biosynthetic pathways (mind the structural similarities)
8 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
elongation by two AcCoA units either decarboxylation can occur leading to the acetonylpyrrolidine hygrine or a secshyond mannich reaction by the intramolecular attack of the acetoacetate anion onto an oxidation‐derived pyrrolinium leading to the tropane skeleton (tropinone) The acetoacetyl‐CoA intermediate can also react intermolecularly with another pyrrolinium cation leading to cuscohygrine after decarboxylation Finally the pyrrolizidine alkaloids [23] are derived from homospermidine which when submitted to terminal oxidative deamination leads to the bicyclic skelshyeton of retronecine and further Senecio alkaloids We can mention herein that ornithine is a biosynthetic precursor of arginine bearing a guanidine function which is an intermediate toward the toxic compounds tetrodotoxin and saxitoxin (not shown)
lysine‐derived alkaloids (incorporating the c5n
unit) From lysine to piperidine alkaloids the biosynthetic steps parallel the one previously described from ornithine Indeed the oxidative deamination of cadaverine affords a δ‐amino aldehyde which cyclizes through imine formation into piperideine Protonation results in a mannich acceptor which is able to react with various nucleophiles such as β‐ketothioester anions The first product of these reactions is pelletierine which can further react through an intramolecular mannich
reaction leading to pseudopelletierine Quinolizidines [24] can also be formed first from the mannich reaction of the piperideine acceptor with the corresponding enamine nucleoshyphile and then after additional transformation steps leading for example to lupinine sparteine or cytisine
Indolizidine alkaloids [15] such as castanospermine and swainsonine are formed from pipecolic acid an amino acid derived from lysine which can be elongated by malonyl‐CoA followed by ring closure When protonated these alkaloids are oxonium mimics strongly inhibiting glycosidases
Tyrosine‐ and Phenylalanine‐Derived Alkaloids Tyrosine and phenylalanine amino acids are bearing the phenylshyethylamine moiety of many medicinally relevant alkaloids Further hydroxylations on the aromatic carbocycle or on the aliphatic part can be observed methylations can occur on phenolic oxygens and on the amine leading to catecholamines (adrenaline noradrenaline dopamine) Arylethylamines are also usual to react with endogenous aldehydes through PictetndashSpengler reactions [25] leading to important biosynthetic intermediates (Scheme 17) like
bull Reticuline from the reaction with 4‐hydroxyphenylacetshyaldehyde toward benzyltetrahydroisoquinoline alkaloids
NH2
Phenylethylamines
RONH TetrahydroisoquinolinesRO
RʹCHO
Rʹ
NH
(CH2)n
MeO
HO
Benzyltetrahydroisoquinoline(n = 1) for example reticuline
orPhenethyltetrahydroisoquinoline
(n = 2) eg autumnaline
IntermolecularCmdashO phenol
couplings
Curarealkaloids
IntramolecularC mdash C phenol
couplings
ONMe
HHO
H
HO
Morphine
NMe
MeO
HO
MeOOH
Isoboldine
O
O
OMe
NO2
CO2H
Aristolochic acid
OMe
O
NHAcMeO
MeOMeO
Colchicine
N
O
O
OMe
OMeBerberine
IntramolecularCmdash C coupling
and furtheroxidative events
Rʹ derivedfrom PHE
Pictet-Spenglerreaction
TYR
HO
HO CHO
HNHO
HO
OH
OMeO
OH
NMe
[O]
GalantamineNorbelladine
Rʹ derived fromsecologanin
NAc
HO
HO
O
CO2MeH
H
H
OHIpecosideaglycone
Ipecac alkaloids
1ClostCmdash C cleavage
and ring extension
H
ROH
1C lost
Cleavage
SCHEME 17 Tyrosine‐derived alkaloid biosynthetic pathways (double head arrows figure bond cleavages during biosynthetic processes)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 9
morphine berberine tubocurarine isoboldine or the highly modified aristolochic acid [26 27]
bull Automnaline from the reaction with 3‐(4‐hydroxyphenyl)propanal toward phenylethyltetrahydroisoquinoline alkaloids colchicine cephalotaxine or schelhammerishycine [28 29]
bull Ipecoside from the reaction of dopamine with secoloshyganin toward terpene tetrahydroisoquinoline alkaloids ipecoside or emetine
Lastly norbelladine (top of Scheme 17) is issued from the reductive amination of 34‐dihydroxybenzaldehyde (derived from phenylalanine) with tyramine (derived from tyrosine) and constitutes a biosynthetic node leading to Amaryllidaceae alkaloids such as galantamine crinine or lycorine depending on the topology of phenolic couplings In all these biosynthetic routes radical phenolic couplings are key reactions for CC and CO bond formations and rearrangements [30 31]
Tryptophan‐Derived Indole and Indole Monoterpene Alkaloids As for alkaloids derived from tyrosine and phenylalanine those derived from tryptophan are formed after decarboxylation of the amino acid (into tryptamine) and possible hydroxylation of the aromatic carbocycle (eg serotonin) and N‐methylation (eg psilocin) As previously tryptamine can also react through PictetndashSpengler reactions to form tetrahydro‐β‐carbolines which can be aromatized for example into harmine (Scheme 18) [16]
When the aldehyde partner of the PictetndashSpengler reacshytion with tryptamine is the terpene secologanin strictosidine is formed as an entry toward the vast monoterpene indole alkaloids [32 33] Hydrolysis of the glucosidic part releases the strictosidine aglycone bearing an aldehyde while iminshyium formation and further cyclization and reduction can lead to ajmalicine (from oxocyclization) or yohimbine (from carshybocyclization) These alkaloids are referred to as from the Corynanthe type with the monoterpene carbon skeleton unmodified Although it misses one carbon and has a very
RO
Carbonylcompounds
TRPNH
NH2
Indolethylamines(eg serotonine)
RONH
NH
Rʹ NH
N
MeOPictet-Spenglerreaction for example Harmineβ-Carbolines
NH
NHH
O
MeO2C
MeO2C
OH
H
H
Rʹ derived from secologanin
Strictosidine aglycone
NH
N
OH
H
H
Ajmalicine
N
N
O
O
H
H
H
Strychnine (C lost)Corynanthe type
Aspidosperma type Iboga type
N
N
OH
OMe
Quinine (C lost)
N
NH CO2Me
Catharanthine
NH
N
CO2MeH
OAc
OH
Vindoline
Complextransformations
NH
NMe
HN
MeN
H
H
Chimonanthine
Cyclizationdimerization
Prenylationcyclization
NH
NMeHO2C
H
Lysergic acid
H
H
Ergot alkaloidsPyrroloindoles Indole monoterpene
1C lost
1C lostFrom AcCoA
SCHEME 18 Tryptophan‐derived alkaloid biosynthetic pathways (gray parts monoterpenic units)
10 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
different structure strychnine is related to the Corynanthe alkaloids incorporating two carbons from acetyl‐CoA Highly modified monoterpene skeletons are derived from the Corynanthe core through CC bond breaking and reorshyganization leading to Iboga‐type (eg catharanthine) and Aspidosperma‐type (eg vindoline) alkaloids The antishycancer drug vinblastine is a heterodimer resulting from the nucleophilic attack of vindoline on a mannich acceptor resulting from catharanthine found in madagascar perishywinkle (Catharanthus roseus) The heteroaromatic comshypounds ellipticine camptothecin and quinine are also derived from a Corynanthe‐type precursor although in this case the biosynthetic relationship may not be obvious due to deep modifications of the skeleton
Finally two important classes of compounds have to be mentioned since they have inspired many synthetic chemists The pyrroloindole alkaloids result from the cyclization of tryptamine as found in physostigmine (formed by a cationic mechanism after methylation in position 3 of the indole not shown) or in chimonanthine (presumably formed by a radical coupling mechanism Scheme 18) The ergot alkaloids are derived from the 33‐dimethylallylation on position 4 of the indole in trypshytophan whose further cyclization and oxidation processes afford the natural products (eg lysergic acid Scheme 18 and ergotamine) which have had important medical applications [34]
NRPS Metabolites and Peptides NRPS enzymes assemble amino acids including nonproteinogenic ones into oligopeptides The enzymes contain several modules and especially an adenylation domain (A) which specifically selects and activates the amino acid to be transferred as a thioester on the nearby peptidyl carrier protein (PCP) [2] A condensation module (C) then catalyzes the formation of the peptide bonds between the newly introduced amino acyl‐PCP (bearing a free amine) and the elongated peptidyl‐PCP thioester At the end of the elongation a cyclization can occur into cyclopeptides but the peptide can also be
transferred to auxiliary enzymes like methyltransferases glycosyltransferases or oxidases (vancomycins are typical products of such functionalizations) [35 36] The formation of heterocycles is also frequently encountered in this metabolism as in penicillins that are derived from the tripeptide α‐aminoadipoyl‐cysteinyl‐valine or telomestatin (Fig 14) [2]
Other Alkaloid Origins There are many other nitrogen sources involved in alkaloid biosyntheses for example nicotinic acid (originated from aspartic acid and intershymediate in nicotine and anabasine biosyntheses) and anthranilic acid (originated from tryptophan and intermedishyate toward acridines or aurachins) The amination reaction (eg through transamination of carbonyl compounds) is also a way to introduce nitrogens in natural products for example from fatty acids steroids (toward Solanum alkaloids or cyclopamines) or other terpenoids (aconitine and atisine have diterpene skeletons while Daphniphyllum alkaloids are triterpene derivatives) Finally nucleic acids can also be precursors of alkaloids like the well‐known caffeine
12 FROM BIOSYNTHESIS TO TOTAL SYNTHESIS STRATEGIES TOWARD THE NATURAL PRODUCT CHEMICAL SPACE
121 The Chemical Space of Natural Products
Natural products occupy an important place in human com munities as demonstrated by their vast use from ancient times to nowadays like dyes fibers oils pershyfumes agrochemicals or drugs Broadly both primary and secondary metabolites could be classified as natural products while the latter as discussed previously are usushyally regarded as the ldquonatural productsrdquo owing to their comshyplexity and diversity arising from a variety of biosynthetic pathways The structural chemical diversity found among
N
S
CO2HO
HNPhH2C
O
Penicillin G
OH
HN
HN
O
O
ONH2
O
OHO
NHMe
OCl
O
NH
NH
NH
O
ClHO
O
HN
H
H
HOOH
OHHO2C
Vancomycin aglycone
ON
NO
O
NN
O
O
N
N O
Me
S
N N
OMeH
Telomestatin
L-AAA-L-CYS-D-VAL
Enzymes
FIGURE 14 Structural diversity of nonribosomal peptide compounds (AAA α‐aminoadipic acid)
FROm BIOSyNTHESIS TO TOTAL SyNTHESIS STRATEGIES TOWARD THE NATuRAL PRODuCT CHEmICAL SPACE 11
all living organisms defining the chemical space of natural products [37] is the consequence of their evolution occurshyring as an adaptation of organisms to their environment It is commonly believed that secondary metabolites are proshyduced as messengers by living organisms or as weapons against enemies and thus they should have certain biological activities in a medicinal point of view [38] Indeed natural products are regarded as one of the main sources of medicines (Fig 15) From the traditional medicinal extracts to every single bioactive molecule the methods of extraction purification identification and biological investigation of natural products have been well established Their complex structures and interesting properties have attracted synthetic chemists to accomplish their total syntheses and that of medicinally relevant anashylogs sometimes in the industrial context [39] Thus tarshygeting the chemical space of natural products has never been more relevant than today Although the discovery of natural products demands time and labor‐consuming manipulations it is worth to notice that the knowledge on this chemical space is still continually growing while biological advances allow for discovering and undershystanding potential targets However increasing the chemical space of human‐made compounds based on natural products should benefit from transdisciplinary colshylaborations such as the use of coupled biosynthetic and chemical synthetic methods to design original ldquounnatural natural productsrdquo [40]
122 The Biosynthetic Pathways as an Inspiration for Synthetic Challenges
1221 Precursor‐Directed Biosyntheses and Mutashysynthetic Strategies to Increase the Chemical Space of Natural Products As the genetics and biochemistry of natural product biosynthesis are better understood novel biosynthetic techniques have been developed to study and generate new diversity in natural product analogs (Scheme 19) Precursor‐directed biosynthesis (PDB) is considered as the earliest example of combining chemical and biological methods for the generation of complex natural product analogs [41 42] This approach compared with the biosynthetic pathway of wild‐type metabolites (Scheme 19a) involves the feeding of analogs of the natural biosynthetic building blocks to the living organisms (Scheme 19b) usually bacteria or fungi which incorporate the modified precursors into the biosynthesized compound mutasynthesis also termed as mutational biosynthesis (mBS) involves the inactivation of a key step of the bioshysynthesis in a mutant microorganism (Scheme 19c) which can then be fed by various modified or advanced building blocks (mutasynthons Scheme 19d) [43] These mutasshyynthons could not be incorporated by the wild type due to specificities of the enzymatic machinery Build up on PDB mBS eliminates the natural biosynthetic intermediate thus generating a less complex mixture of metabolites and making the purification or yield of target products better
O NH
OHO
O
OO
O
O
OH
HOO
O O
OH
HOO
NH ONH2
O
ONH
NH
O
OCl Cl
O
O
OH
HN
HN
HN
HN
OO
HO
HN
OH
OHOH
O
O
HO
HO
OHO
O
OHH2N
O
OH3C
H
O
H
CH3
CH3
H
O O
NO
H
O
HO
O
NO
O
NH
HN
OHO
HONH
ON
H
HO
O
H2N
O OH
ONH
OH
ON OHO
OH
HO OS OH
OOOH
OHO
O
ON
OO
O
HO
O
O OH
OO
Micafunginantifungal
Rapamycinimmunosuppressive
Galantamineanti-Alzheimer
Taxolanticancer
Artemisininantimalarial
Vancomycinantibiotic
FIGURE 15 Some famous natural products currently used as drugs
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 3
chloroplast mEP is a starting block in the biosynthesis of terpenes through C
5 isoprene units (isopentenyl diphosphate
(IPP) and dimethylallyl diphosphate (DmAPP)) especially those in C
10 C
20 and C
40 terpenes 3‐PGA is a precursor of
serine and other amino acids while phosphoenolpyruvate (PEP) the precursor of PA is also an intermediate toward the previously mentioned shikimic acid Lastly PA is not only a precursor of the fundamental ldquoC
2rdquo acetyl coenzyme A
(AcCoA) unit but also an intermediate toward aliphatic amino acids and mEP
AcCoA is the building block of fatty acids polyketides and mevalonic acid (mVA) a cytosolic precursor of the C
5 isoprene units for the biosynthesis of terpenes in the
C15
and C30
series (mind it is different from the mEP pathway in product and in cell location) Finally AcCoA enters the citric acid or Krebs cycle which leads to several precursors of amino acids These are oxaloacetic acid precursor of aspartic acid through transamination (thus toward lysine as a nitrogenated C
5N linear unit and methishy
onine as a methyl supplier) and 2‐oxoglutaric acid preshycursor of glutamic acid (and subsequent derivatives such as ornithine as a nitrogenated C
4N linear unit) All these
amino acids are key precursors in the biosynthesis of many alkaloids
114 Reactions Involved in the Construction of Secondary Metabolites
most reactions occurring in the living cells are performed by specialized enzymes which have been classified in an intershynational nomenclature defined by an enzyme commission (EC) number There are six classes of enzymes depending on the biochemical reaction they catalyze EC‐1 oxidoreducshytases (catalyzing oxidoreduction reactions) EC‐2 transfershyases (catalyzing the transfer of functional groups) EC‐3 hydrolases (catalyzing hydrolysis) EC‐4 lyases (breaking bonds through another process than hydrolysis or oxidation leading to a new double bond or a new cycle) EC‐5 isomershyases (catalyzing the isomerization of a molecule) and EC‐6 ligases (forming a covalent bond between two molecules) many subclasses of these enzymes have been described depending on the type of atoms and functional groups involved in the reaction and if any on the cofactor used in this reaction For example several cofactors can be used by dehydrogenases like NAD(P)NAD(P)H FADFADH
2 or
FmNFmNH2 For a description of this classification the
reader can refer to specialized Internet websites like ExplorEnz [1] What is important to realize is that most enzymes are substrate specific and have been selected during
CHOHO
CO2H
O
CH2OPO32ndash
CH2OPO32ndash CH2OPO3
2ndash
HOOH
HO
HOH2CO
OHC
CH2OPO32ndash
OHHO
CO2H
HO
OH
OH
TYR
PHE
CO2H
NH2
TRP
CH2OPO32ndash
HO
HOH2COH
MEP
C10 C20 and C40 terpenes
CO2HHO
SER
GLY CYS
CO2HOPO3
2ndash
CH2OH
OHHO2C
MVA
C15 and C30 terpenes
Polyketides
Krebscycle
(citric acid)
VAL ALA ILE LEU
CO2H
O
HO2C
CO2HO
CO2H
ASP
LYS
GLU
Glucose-6-phosphate
Fructose-6-phosphate
Erythrose-4-phosphate
3-PGAL
OP2O63ndash OP2O6
3ndash
IPP DMAPP
3-PGA PEP PA
O SCoAAcCoA
3x
Cytosol
Cytosol
Chloroplasts
Chloroplasts
Glycolysis
C2
C5
Shikimic acid Oxaloaceticacid
2-Oxoglutaricacid
MET
THR
PRO
ARGORN
Alkaloids Alkaloids Alkaloids Alkaloids
Aminoacids
Peptidesproteins
Phenolics
C1
C5N C4N
ldquoCH3rdquo
(Indole)C2NC6C3C6C2N C6C1
Pentosephosphatepathway
SCHEME 12 The building block chart involving glycolysis and the Krebs cycle
4 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
evolution to perform specific transformations making natural products with often and yet unknown functions
Secondary metabolites arise from specific biosynthetic pathways which use the previously defined building blocks The bunch of organic reactions involved in these biosynshytheses allows the construction of natural product frameshyworks which are finally diversified through ldquodecorationrdquo steps (Scheme 13) It is not the purpose of this introductive chapter to describe in detail all biosynthetic pathways and the reader can refer to excellent books and articles which have been published elsewhere [2 3]
The reactions involved in the construction of natural product skeletons will be described later for representative classes of compounds The identification of the building block footprint in the natural product skeleton will be emphasized as much as possible sometimes referring to biogenetic speculations [4] After the framework construction the decoration steps will involve as diverse reactions as aliphatic CH oxidations (eg involving a cytochrome P
450 oxygenase) occasionally triggering a
rearrangement heteroatom alkylations (eg methylation by S‐adenosylmethionine) or allylation (by DmAPP) esterifications heteroatom or C‐glycosylations (leading to heterosides) radical couplings (especially for phenols) alcohol oxidations or ketone reductions amineketone transaminations alkene dihydroxylations or epoxidations oxidative halogenations BaeyerndashVilliger oxidations and further oxygenation steps At the end of the biosynthesis such transformations may totally hide the primary building block origin of natural products
115 Secondary Metabolisms
1151 Polyketides Polyketides (or polyacetates) are issued from the oligomerization of C
2 acetate units performed
by polyketide synthases (PKS) and leading to (C2)
n linear
intermediates [5 6] If the (C2)
n intermediates arise from
successive Claisen reactions performed by ketosynthase
domains (KS in nonreducing PKS) a highly reactive poly‐ β‐ketoacyl intermediate H(CH
2CO)
nOH is formed
leading to phenolic and aromatic products through further intramolecular Claisen condensations Furthermore highly reducing PKSs are made of specialized enzymatic subunits working in line or iteratively to functionalize each C
2 linker
bond as CH(OH)CH2 (by ketoreductases (KR)) then as
HCCH (by dehydratases (DH)) and as CH2CH
2 (by enoyl
reductases (ER)) leading to a high degree of functionalization of the final product (Fig 12) By these iterative sequences highly reduced polyketides which can be either linear macrocyclized or polycyclized depending on the reactivity of the biosynthetic intermediates can be formed [7] With the same logic fatty acids are also biosynthesized by fatty acid synthases
moreover the PKS enzyme can be hybridized with nonshyribosomal peptide synthetase (NRPS) domains (see also ldquoNRPS metabolites and peptidesrdquo in the ldquoAlkaloidsrdquo secshytion) leading to the acylation of an amino acid by the (C
2)
n
acyl intermediate As previously the functionalization of the acyl chain depends on the PKS enzyme and the PKSNRPS products are also extremely diversified (eg hirsutellone B Fig 12) [8]
1152 Terpenes Terpenes are derived from the oligoshymerization of the C
5 isoprene units DmAPP and IPP Both
precursors are prompt to generate either an allylic cation (the diphosphate is a good leaving group) or a tertiary carbocation respectively which makes the IPP easy to react with DmAPP (Scheme 14) This reaction happens in the active site of a terpene synthase which activates the deparshyture of the diphosphate group from DmAPP thanks to Lewis acid activation (a metal like mg2+ mn2+ or Co2+ is present in the enzyme active site [9]) This leads to geranyl (C
10
monoterpene precursor) or farnesyl (C15
sesquiterpene precursor) diphosphate depending on the location of the enzyme (chloroplast for the mEP pathway or cytosol for the mVA pathway) Geranylgeranyl (C
20 diterpene) and
Constructionreactions
Decoration reaction(functionalization)Building
blocksNatural product
frameworksNatural products
HH
OH OAcH
HO O OH
OHO
OBz
P450
P450
P450
P450
P450 P450 Oxidativeauxiliaryenzymes
Taxadiene
OP2O63ndash
OP2O63ndash
3times
(a)
(b)
10-Deacetylbaccatin III
DMAPP
IPPand electrophilic
cyclizations
SCHEME 13 (a) From building blocks to natural products and (b) the example of 10‐deacetylbaccatin III
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 5
farnesylfarnesyl (C30
triterpene precursor) diphosphates can also be obtained by further additions of IPP leading to longer linear intermediates
The cyclization of linear precursors is achieved by speshycialized cyclases which generate a poorly functionalized natural product framework [10 11] Auxiliary enzymes such as oxygenases then increase the complexity and the diversity
of compounds by further functionalization (Scheme 13b) [12] A high degree of oxidation can be observed in comshypounds like thapsigargin paclitaxel or bilobalide (Fig 13) The biosynthesis of this last compound for example involves a high oxygenation pattern two Wagnerndashmeerwein rearrangements and several oxidative cleavages leading to the loss of five carbons The resulting natural products can
KS
S-ACP
O O
KR
S-ACP
OH O
S-MAT
O
YesNo
R
R
R
YesNoDH
S-ACP
OR
YesNoER
S-ACP
OR
YesNoTE
OH
OR
New cycle
New cycleor
release
New cycleor
release
New cycleor
release
Release
R in
crem
ente
d by
two
carb
ons
HO
O
S-ACP
O
Claisen condensation (ndashCO2)
reduction (NADPH)
dehydration (ndashH2O)
+
reduction (NADPH)
hydrolysis (H2O)
O O NH
OH
OH
H H
H H
Hirsutellone B(mixed PKSNRPS
product)
OO
O
OHOH
HO OH
Norsolorinic acid
O
O
O
O CHO
OH
HH
H
H
HH
OHHemibrevetoxin B
OOHO
OHO
HOHO OH
Erythronolide A
OH
OH
HO
Panaxytriol
From ahexanoylstartingblock
H
H
Fromtyrosine
H
1C lost fromdecarboxylation
FIGURE 12 Chemical logic of polyketide construction leading to variable functionalization of the elongated acyl chain and examples of resulting chemical diversity ACP acyl carrier protein DH dehydratase ER enoyl reductase KR ketoreductase KS ketosynthase mAT malonyl acyl transferase TE thioesterase
DMAPPOP2O6
3ndashOP2O6
3ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
Geranyl diphosphate (GPP)2 times IPP
H H
Geranylgeranyl diphosphate (GGPP)
Examplesverticillane
casbanetaxane
phorbol
Exampleslabdane
pimaranekauraneabietane
aphidicolanegibberellane
IPP
SCHEME 14 Early assembly of C5 units in terpene biosynthesis leading to diterpenes (C
20)
6 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
thus be extremely modified with structures whose biogenetic origin is far from being obvious at first sight and cannot be determined without further experiments such as isotopic labeling
1153 Flavonoids Resveratrols Gallic Acids and Further Polyphenolics We have previously discussed the polyketide origin of some phenolic compounds based on the (C
2)
n motif Other polyphenols like gallic acids are directly
derived by the aromatization of shikimic acid (C6C
1 building
block Scheme 12) [13] The C6C
3 building blocks are availshy
able from the conversion of phenylalanine and tyrosine into cinnamic and p‐coumaric acids respectively and then by further hydroxylation steps (Scheme 15) These can dimerize into lignans (eg podophyllotoxin) [14 15] through radical processes or converted to low molecular weight compounds like eugenol coumarins or vanillin [16] The coenzyme A thioesters of these C
6C
3 acids can be used as initiator units
by specialized ketosynthases for an elongation by two acetyl units leading to aromatic polyketides like styrylpyrones or diarylheptanoids (eg curcumin) [17] Important comshypounds from this metabolism are flavonoids (C
6C
3C
6) [18]
and stilbenoids (C6C
2C
6) (a decarboxylation occurs during
the aryl cyclization) [19] which are synthesized by chalcone synthase and stilbene synthase respectively Flavonoids (eg catechin) and stilbenes (eg resveratrol) are present in large amounts in fruits and vegetables and may exert their radical scavenging properties in vivo
1154 Alkaloids Alkaloids are nitrogen‐containing compounds The nitrogen(s) can be involved in an amine function conferring basicity to the natural product (like ldquoalkalirdquo) or in less or nonbasic functions such as an amide a nitrile an isonitrile or an ammonium salt (quaternary amines) For amines protonation often occurs at physiological pH and may condition their biological activity In many cases the nitrogen is biogenetically derived from an amino acid We will thus discuss alkaloids according to their amino acid origin
Alkaloids Derived from the Krebs Cycle (Lysine and Ornithine Derived) As shown previously (Scheme 12) the Krebs cycle is a crucial metabolic process which leads to α‐ketoacids (oxaloacetic and 2‐oxoglutaric acids) Their enzymatic transamination affords the two amino acidsmdashaspartic acid and glutamic acidmdashwhich are the direct biosynthetic precursors of amino acids lysine and ornithine respectively These in turn produce cadaverine a ldquoC
5Nrdquo unit
and putrescine a ldquoC4Nrdquo unit which are major components
for the biosynthesis of important alkaloids as will be discussed later (Scheme 16) Additionally ornithine is a precursor of arginine another important amino acid
ornithine‐derived alkaloids (incorporating the c4n
unit) Putrescine is derived from the decarboxylation of ornithine and is a precursor of linear polyamines like spermshyine After enzymatic methylation of one amine of putrescine
C10 C15
C15
C10
C10
C30
CO2H
Chrysanthemic acid
OH
Menthol
O
HO
H
HOGlc
MeO2CLoganin (iridoid)
H
H
H
HH
OO
OParthenolide
O
O
OHOHO
OAcHO
OnC3H7
O
O
O
nC7H15
Thapsigargin
O
O
OO
H
H
O
Cleavage
H
H
Artemisinin
C20 C40
O
O
OO
OO
OHOHH
Bilobalide(highly rearrangedand missing 5 C)
CleavageH
Highoxidativecleavages
HO OAcH
AcO O OH
OOOBz
O
Ph
BzNH
OHFrom PHE Paclitaxel
C25O
H
HO
OHCH
OH Ophiobolin A
H
HO
H
H
HHCholesterol
(missing 3 C WM)
H
H H H
H
H
Hopene
β-Carotene
C30 - 3
C15
C20 - 5WM
WM
FIGURE 13 Chemical diversity in the terpene series (Wm Wagnerndashmeerwein shifts lost carbons bold bonds are remnant of primary building blocks)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 7
in the presence of S‐adenosylmethionine transamination of the other affords γ‐(N‐methylamino)aldehyde [20] The resulting cyclic iminium is a key intermediate in the formation of many medicinally important alkaloids such as
the plant‐derived compounds cocaine atropine or the calysshytegines [21 22] Indeed this iminium is a mannich acceptor which can react with various nucleophiles the first of those being the carbanion of acetyl‐CoA Thus after a stepwise
CO2H
RCinnamic acid (R=H)p-Coumaric acid (R=OH)
RO
PHETYR
OH
[H] [O]
lignans
OH
O
O
O
O
OMeMeO
OMe
C mdash C andor C mdash Oradical couplings
After E Zisomerization
for example Podophyllotoxin
O ORO
Coumarins(eg scopoletin)
[O]
Prenylationcyclizationcleavage[O]
O OO
Furocoumarins(eg psoralen)
RO
nAcetyl-CoA
thencyclization
n = 2 styrylpyrones
O O
OMe
n = 3 avonoids(from chalcone synthase)
C6C3
H
H H
From AcCoA
O
n = 3 stilbenoids(from stilbene synthase)
HO
OH
OHFrom AcCoA From AcCoA(ndashCO2)
Resveratrol
HO
OH
OH
OH
OH
H
Catechin
MeO Yangonin
From AcCoA
SCHEME 15 The phenylpropanoid biosynthetic pathways
LYS
CO2
Cadaverine
O NH
H2O
NH
Piperideineiminium
Pelletierine
AcAcCoA
CO2
NH
O
N
O
Pseudopelletierine
NH
Ph
OH
Ph
O
Lobeline
NH
δ+
δndash
N
H
H
N
H
OH
Lupinine
N
H N
H
(+)-Sparteine
N
NH
O (ndash)-Cytisine
NH
CO2H
Pipecolic acid
N
OHHO
OH
HO
H
Castanos permine
ORN
CO2
NH2 NH2 NH2 NH2
NH2
NH2
NH2
PutrescineO
NHMe
N-Methylpyrrolinium
2 timesAcCoA
NMe
ARG
n = 1 spermidinen = 2 homospermidine
NMe
O
HygrineCO2
CO2
MeN
O
[O]
TropinoneCO2
HNHO
OHOH
OH
Calystegine B2
MeN
OAtropine
O
Ph
HO
NMe
NMe
NMe
O
Cuscohygrine
HN
HN
NH2
( )n
O
n = 2
N
HHO
Retronecine
HH
H
H
H
HO
H H
H
SCHEME 16 Lysine‐ and ornithine‐derived alkaloid biosynthetic pathways (mind the structural similarities)
8 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
elongation by two AcCoA units either decarboxylation can occur leading to the acetonylpyrrolidine hygrine or a secshyond mannich reaction by the intramolecular attack of the acetoacetate anion onto an oxidation‐derived pyrrolinium leading to the tropane skeleton (tropinone) The acetoacetyl‐CoA intermediate can also react intermolecularly with another pyrrolinium cation leading to cuscohygrine after decarboxylation Finally the pyrrolizidine alkaloids [23] are derived from homospermidine which when submitted to terminal oxidative deamination leads to the bicyclic skelshyeton of retronecine and further Senecio alkaloids We can mention herein that ornithine is a biosynthetic precursor of arginine bearing a guanidine function which is an intermediate toward the toxic compounds tetrodotoxin and saxitoxin (not shown)
lysine‐derived alkaloids (incorporating the c5n
unit) From lysine to piperidine alkaloids the biosynthetic steps parallel the one previously described from ornithine Indeed the oxidative deamination of cadaverine affords a δ‐amino aldehyde which cyclizes through imine formation into piperideine Protonation results in a mannich acceptor which is able to react with various nucleophiles such as β‐ketothioester anions The first product of these reactions is pelletierine which can further react through an intramolecular mannich
reaction leading to pseudopelletierine Quinolizidines [24] can also be formed first from the mannich reaction of the piperideine acceptor with the corresponding enamine nucleoshyphile and then after additional transformation steps leading for example to lupinine sparteine or cytisine
Indolizidine alkaloids [15] such as castanospermine and swainsonine are formed from pipecolic acid an amino acid derived from lysine which can be elongated by malonyl‐CoA followed by ring closure When protonated these alkaloids are oxonium mimics strongly inhibiting glycosidases
Tyrosine‐ and Phenylalanine‐Derived Alkaloids Tyrosine and phenylalanine amino acids are bearing the phenylshyethylamine moiety of many medicinally relevant alkaloids Further hydroxylations on the aromatic carbocycle or on the aliphatic part can be observed methylations can occur on phenolic oxygens and on the amine leading to catecholamines (adrenaline noradrenaline dopamine) Arylethylamines are also usual to react with endogenous aldehydes through PictetndashSpengler reactions [25] leading to important biosynthetic intermediates (Scheme 17) like
bull Reticuline from the reaction with 4‐hydroxyphenylacetshyaldehyde toward benzyltetrahydroisoquinoline alkaloids
NH2
Phenylethylamines
RONH TetrahydroisoquinolinesRO
RʹCHO
Rʹ
NH
(CH2)n
MeO
HO
Benzyltetrahydroisoquinoline(n = 1) for example reticuline
orPhenethyltetrahydroisoquinoline
(n = 2) eg autumnaline
IntermolecularCmdashO phenol
couplings
Curarealkaloids
IntramolecularC mdash C phenol
couplings
ONMe
HHO
H
HO
Morphine
NMe
MeO
HO
MeOOH
Isoboldine
O
O
OMe
NO2
CO2H
Aristolochic acid
OMe
O
NHAcMeO
MeOMeO
Colchicine
N
O
O
OMe
OMeBerberine
IntramolecularCmdash C coupling
and furtheroxidative events
Rʹ derivedfrom PHE
Pictet-Spenglerreaction
TYR
HO
HO CHO
HNHO
HO
OH
OMeO
OH
NMe
[O]
GalantamineNorbelladine
Rʹ derived fromsecologanin
NAc
HO
HO
O
CO2MeH
H
H
OHIpecosideaglycone
Ipecac alkaloids
1ClostCmdash C cleavage
and ring extension
H
ROH
1C lost
Cleavage
SCHEME 17 Tyrosine‐derived alkaloid biosynthetic pathways (double head arrows figure bond cleavages during biosynthetic processes)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 9
morphine berberine tubocurarine isoboldine or the highly modified aristolochic acid [26 27]
bull Automnaline from the reaction with 3‐(4‐hydroxyphenyl)propanal toward phenylethyltetrahydroisoquinoline alkaloids colchicine cephalotaxine or schelhammerishycine [28 29]
bull Ipecoside from the reaction of dopamine with secoloshyganin toward terpene tetrahydroisoquinoline alkaloids ipecoside or emetine
Lastly norbelladine (top of Scheme 17) is issued from the reductive amination of 34‐dihydroxybenzaldehyde (derived from phenylalanine) with tyramine (derived from tyrosine) and constitutes a biosynthetic node leading to Amaryllidaceae alkaloids such as galantamine crinine or lycorine depending on the topology of phenolic couplings In all these biosynthetic routes radical phenolic couplings are key reactions for CC and CO bond formations and rearrangements [30 31]
Tryptophan‐Derived Indole and Indole Monoterpene Alkaloids As for alkaloids derived from tyrosine and phenylalanine those derived from tryptophan are formed after decarboxylation of the amino acid (into tryptamine) and possible hydroxylation of the aromatic carbocycle (eg serotonin) and N‐methylation (eg psilocin) As previously tryptamine can also react through PictetndashSpengler reactions to form tetrahydro‐β‐carbolines which can be aromatized for example into harmine (Scheme 18) [16]
When the aldehyde partner of the PictetndashSpengler reacshytion with tryptamine is the terpene secologanin strictosidine is formed as an entry toward the vast monoterpene indole alkaloids [32 33] Hydrolysis of the glucosidic part releases the strictosidine aglycone bearing an aldehyde while iminshyium formation and further cyclization and reduction can lead to ajmalicine (from oxocyclization) or yohimbine (from carshybocyclization) These alkaloids are referred to as from the Corynanthe type with the monoterpene carbon skeleton unmodified Although it misses one carbon and has a very
RO
Carbonylcompounds
TRPNH
NH2
Indolethylamines(eg serotonine)
RONH
NH
Rʹ NH
N
MeOPictet-Spenglerreaction for example Harmineβ-Carbolines
NH
NHH
O
MeO2C
MeO2C
OH
H
H
Rʹ derived from secologanin
Strictosidine aglycone
NH
N
OH
H
H
Ajmalicine
N
N
O
O
H
H
H
Strychnine (C lost)Corynanthe type
Aspidosperma type Iboga type
N
N
OH
OMe
Quinine (C lost)
N
NH CO2Me
Catharanthine
NH
N
CO2MeH
OAc
OH
Vindoline
Complextransformations
NH
NMe
HN
MeN
H
H
Chimonanthine
Cyclizationdimerization
Prenylationcyclization
NH
NMeHO2C
H
Lysergic acid
H
H
Ergot alkaloidsPyrroloindoles Indole monoterpene
1C lost
1C lostFrom AcCoA
SCHEME 18 Tryptophan‐derived alkaloid biosynthetic pathways (gray parts monoterpenic units)
10 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
different structure strychnine is related to the Corynanthe alkaloids incorporating two carbons from acetyl‐CoA Highly modified monoterpene skeletons are derived from the Corynanthe core through CC bond breaking and reorshyganization leading to Iboga‐type (eg catharanthine) and Aspidosperma‐type (eg vindoline) alkaloids The antishycancer drug vinblastine is a heterodimer resulting from the nucleophilic attack of vindoline on a mannich acceptor resulting from catharanthine found in madagascar perishywinkle (Catharanthus roseus) The heteroaromatic comshypounds ellipticine camptothecin and quinine are also derived from a Corynanthe‐type precursor although in this case the biosynthetic relationship may not be obvious due to deep modifications of the skeleton
Finally two important classes of compounds have to be mentioned since they have inspired many synthetic chemists The pyrroloindole alkaloids result from the cyclization of tryptamine as found in physostigmine (formed by a cationic mechanism after methylation in position 3 of the indole not shown) or in chimonanthine (presumably formed by a radical coupling mechanism Scheme 18) The ergot alkaloids are derived from the 33‐dimethylallylation on position 4 of the indole in trypshytophan whose further cyclization and oxidation processes afford the natural products (eg lysergic acid Scheme 18 and ergotamine) which have had important medical applications [34]
NRPS Metabolites and Peptides NRPS enzymes assemble amino acids including nonproteinogenic ones into oligopeptides The enzymes contain several modules and especially an adenylation domain (A) which specifically selects and activates the amino acid to be transferred as a thioester on the nearby peptidyl carrier protein (PCP) [2] A condensation module (C) then catalyzes the formation of the peptide bonds between the newly introduced amino acyl‐PCP (bearing a free amine) and the elongated peptidyl‐PCP thioester At the end of the elongation a cyclization can occur into cyclopeptides but the peptide can also be
transferred to auxiliary enzymes like methyltransferases glycosyltransferases or oxidases (vancomycins are typical products of such functionalizations) [35 36] The formation of heterocycles is also frequently encountered in this metabolism as in penicillins that are derived from the tripeptide α‐aminoadipoyl‐cysteinyl‐valine or telomestatin (Fig 14) [2]
Other Alkaloid Origins There are many other nitrogen sources involved in alkaloid biosyntheses for example nicotinic acid (originated from aspartic acid and intershymediate in nicotine and anabasine biosyntheses) and anthranilic acid (originated from tryptophan and intermedishyate toward acridines or aurachins) The amination reaction (eg through transamination of carbonyl compounds) is also a way to introduce nitrogens in natural products for example from fatty acids steroids (toward Solanum alkaloids or cyclopamines) or other terpenoids (aconitine and atisine have diterpene skeletons while Daphniphyllum alkaloids are triterpene derivatives) Finally nucleic acids can also be precursors of alkaloids like the well‐known caffeine
12 FROM BIOSYNTHESIS TO TOTAL SYNTHESIS STRATEGIES TOWARD THE NATURAL PRODUCT CHEMICAL SPACE
121 The Chemical Space of Natural Products
Natural products occupy an important place in human com munities as demonstrated by their vast use from ancient times to nowadays like dyes fibers oils pershyfumes agrochemicals or drugs Broadly both primary and secondary metabolites could be classified as natural products while the latter as discussed previously are usushyally regarded as the ldquonatural productsrdquo owing to their comshyplexity and diversity arising from a variety of biosynthetic pathways The structural chemical diversity found among
N
S
CO2HO
HNPhH2C
O
Penicillin G
OH
HN
HN
O
O
ONH2
O
OHO
NHMe
OCl
O
NH
NH
NH
O
ClHO
O
HN
H
H
HOOH
OHHO2C
Vancomycin aglycone
ON
NO
O
NN
O
O
N
N O
Me
S
N N
OMeH
Telomestatin
L-AAA-L-CYS-D-VAL
Enzymes
FIGURE 14 Structural diversity of nonribosomal peptide compounds (AAA α‐aminoadipic acid)
FROm BIOSyNTHESIS TO TOTAL SyNTHESIS STRATEGIES TOWARD THE NATuRAL PRODuCT CHEmICAL SPACE 11
all living organisms defining the chemical space of natural products [37] is the consequence of their evolution occurshyring as an adaptation of organisms to their environment It is commonly believed that secondary metabolites are proshyduced as messengers by living organisms or as weapons against enemies and thus they should have certain biological activities in a medicinal point of view [38] Indeed natural products are regarded as one of the main sources of medicines (Fig 15) From the traditional medicinal extracts to every single bioactive molecule the methods of extraction purification identification and biological investigation of natural products have been well established Their complex structures and interesting properties have attracted synthetic chemists to accomplish their total syntheses and that of medicinally relevant anashylogs sometimes in the industrial context [39] Thus tarshygeting the chemical space of natural products has never been more relevant than today Although the discovery of natural products demands time and labor‐consuming manipulations it is worth to notice that the knowledge on this chemical space is still continually growing while biological advances allow for discovering and undershystanding potential targets However increasing the chemical space of human‐made compounds based on natural products should benefit from transdisciplinary colshylaborations such as the use of coupled biosynthetic and chemical synthetic methods to design original ldquounnatural natural productsrdquo [40]
122 The Biosynthetic Pathways as an Inspiration for Synthetic Challenges
1221 Precursor‐Directed Biosyntheses and Mutashysynthetic Strategies to Increase the Chemical Space of Natural Products As the genetics and biochemistry of natural product biosynthesis are better understood novel biosynthetic techniques have been developed to study and generate new diversity in natural product analogs (Scheme 19) Precursor‐directed biosynthesis (PDB) is considered as the earliest example of combining chemical and biological methods for the generation of complex natural product analogs [41 42] This approach compared with the biosynthetic pathway of wild‐type metabolites (Scheme 19a) involves the feeding of analogs of the natural biosynthetic building blocks to the living organisms (Scheme 19b) usually bacteria or fungi which incorporate the modified precursors into the biosynthesized compound mutasynthesis also termed as mutational biosynthesis (mBS) involves the inactivation of a key step of the bioshysynthesis in a mutant microorganism (Scheme 19c) which can then be fed by various modified or advanced building blocks (mutasynthons Scheme 19d) [43] These mutasshyynthons could not be incorporated by the wild type due to specificities of the enzymatic machinery Build up on PDB mBS eliminates the natural biosynthetic intermediate thus generating a less complex mixture of metabolites and making the purification or yield of target products better
O NH
OHO
O
OO
O
O
OH
HOO
O O
OH
HOO
NH ONH2
O
ONH
NH
O
OCl Cl
O
O
OH
HN
HN
HN
HN
OO
HO
HN
OH
OHOH
O
O
HO
HO
OHO
O
OHH2N
O
OH3C
H
O
H
CH3
CH3
H
O O
NO
H
O
HO
O
NO
O
NH
HN
OHO
HONH
ON
H
HO
O
H2N
O OH
ONH
OH
ON OHO
OH
HO OS OH
OOOH
OHO
O
ON
OO
O
HO
O
O OH
OO
Micafunginantifungal
Rapamycinimmunosuppressive
Galantamineanti-Alzheimer
Taxolanticancer
Artemisininantimalarial
Vancomycinantibiotic
FIGURE 15 Some famous natural products currently used as drugs
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product
4 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
evolution to perform specific transformations making natural products with often and yet unknown functions
Secondary metabolites arise from specific biosynthetic pathways which use the previously defined building blocks The bunch of organic reactions involved in these biosynshytheses allows the construction of natural product frameshyworks which are finally diversified through ldquodecorationrdquo steps (Scheme 13) It is not the purpose of this introductive chapter to describe in detail all biosynthetic pathways and the reader can refer to excellent books and articles which have been published elsewhere [2 3]
The reactions involved in the construction of natural product skeletons will be described later for representative classes of compounds The identification of the building block footprint in the natural product skeleton will be emphasized as much as possible sometimes referring to biogenetic speculations [4] After the framework construction the decoration steps will involve as diverse reactions as aliphatic CH oxidations (eg involving a cytochrome P
450 oxygenase) occasionally triggering a
rearrangement heteroatom alkylations (eg methylation by S‐adenosylmethionine) or allylation (by DmAPP) esterifications heteroatom or C‐glycosylations (leading to heterosides) radical couplings (especially for phenols) alcohol oxidations or ketone reductions amineketone transaminations alkene dihydroxylations or epoxidations oxidative halogenations BaeyerndashVilliger oxidations and further oxygenation steps At the end of the biosynthesis such transformations may totally hide the primary building block origin of natural products
115 Secondary Metabolisms
1151 Polyketides Polyketides (or polyacetates) are issued from the oligomerization of C
2 acetate units performed
by polyketide synthases (PKS) and leading to (C2)
n linear
intermediates [5 6] If the (C2)
n intermediates arise from
successive Claisen reactions performed by ketosynthase
domains (KS in nonreducing PKS) a highly reactive poly‐ β‐ketoacyl intermediate H(CH
2CO)
nOH is formed
leading to phenolic and aromatic products through further intramolecular Claisen condensations Furthermore highly reducing PKSs are made of specialized enzymatic subunits working in line or iteratively to functionalize each C
2 linker
bond as CH(OH)CH2 (by ketoreductases (KR)) then as
HCCH (by dehydratases (DH)) and as CH2CH
2 (by enoyl
reductases (ER)) leading to a high degree of functionalization of the final product (Fig 12) By these iterative sequences highly reduced polyketides which can be either linear macrocyclized or polycyclized depending on the reactivity of the biosynthetic intermediates can be formed [7] With the same logic fatty acids are also biosynthesized by fatty acid synthases
moreover the PKS enzyme can be hybridized with nonshyribosomal peptide synthetase (NRPS) domains (see also ldquoNRPS metabolites and peptidesrdquo in the ldquoAlkaloidsrdquo secshytion) leading to the acylation of an amino acid by the (C
2)
n
acyl intermediate As previously the functionalization of the acyl chain depends on the PKS enzyme and the PKSNRPS products are also extremely diversified (eg hirsutellone B Fig 12) [8]
1152 Terpenes Terpenes are derived from the oligoshymerization of the C
5 isoprene units DmAPP and IPP Both
precursors are prompt to generate either an allylic cation (the diphosphate is a good leaving group) or a tertiary carbocation respectively which makes the IPP easy to react with DmAPP (Scheme 14) This reaction happens in the active site of a terpene synthase which activates the deparshyture of the diphosphate group from DmAPP thanks to Lewis acid activation (a metal like mg2+ mn2+ or Co2+ is present in the enzyme active site [9]) This leads to geranyl (C
10
monoterpene precursor) or farnesyl (C15
sesquiterpene precursor) diphosphate depending on the location of the enzyme (chloroplast for the mEP pathway or cytosol for the mVA pathway) Geranylgeranyl (C
20 diterpene) and
Constructionreactions
Decoration reaction(functionalization)Building
blocksNatural product
frameworksNatural products
HH
OH OAcH
HO O OH
OHO
OBz
P450
P450
P450
P450
P450 P450 Oxidativeauxiliaryenzymes
Taxadiene
OP2O63ndash
OP2O63ndash
3times
(a)
(b)
10-Deacetylbaccatin III
DMAPP
IPPand electrophilic
cyclizations
SCHEME 13 (a) From building blocks to natural products and (b) the example of 10‐deacetylbaccatin III
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 5
farnesylfarnesyl (C30
triterpene precursor) diphosphates can also be obtained by further additions of IPP leading to longer linear intermediates
The cyclization of linear precursors is achieved by speshycialized cyclases which generate a poorly functionalized natural product framework [10 11] Auxiliary enzymes such as oxygenases then increase the complexity and the diversity
of compounds by further functionalization (Scheme 13b) [12] A high degree of oxidation can be observed in comshypounds like thapsigargin paclitaxel or bilobalide (Fig 13) The biosynthesis of this last compound for example involves a high oxygenation pattern two Wagnerndashmeerwein rearrangements and several oxidative cleavages leading to the loss of five carbons The resulting natural products can
KS
S-ACP
O O
KR
S-ACP
OH O
S-MAT
O
YesNo
R
R
R
YesNoDH
S-ACP
OR
YesNoER
S-ACP
OR
YesNoTE
OH
OR
New cycle
New cycleor
release
New cycleor
release
New cycleor
release
Release
R in
crem
ente
d by
two
carb
ons
HO
O
S-ACP
O
Claisen condensation (ndashCO2)
reduction (NADPH)
dehydration (ndashH2O)
+
reduction (NADPH)
hydrolysis (H2O)
O O NH
OH
OH
H H
H H
Hirsutellone B(mixed PKSNRPS
product)
OO
O
OHOH
HO OH
Norsolorinic acid
O
O
O
O CHO
OH
HH
H
H
HH
OHHemibrevetoxin B
OOHO
OHO
HOHO OH
Erythronolide A
OH
OH
HO
Panaxytriol
From ahexanoylstartingblock
H
H
Fromtyrosine
H
1C lost fromdecarboxylation
FIGURE 12 Chemical logic of polyketide construction leading to variable functionalization of the elongated acyl chain and examples of resulting chemical diversity ACP acyl carrier protein DH dehydratase ER enoyl reductase KR ketoreductase KS ketosynthase mAT malonyl acyl transferase TE thioesterase
DMAPPOP2O6
3ndashOP2O6
3ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
Geranyl diphosphate (GPP)2 times IPP
H H
Geranylgeranyl diphosphate (GGPP)
Examplesverticillane
casbanetaxane
phorbol
Exampleslabdane
pimaranekauraneabietane
aphidicolanegibberellane
IPP
SCHEME 14 Early assembly of C5 units in terpene biosynthesis leading to diterpenes (C
20)
6 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
thus be extremely modified with structures whose biogenetic origin is far from being obvious at first sight and cannot be determined without further experiments such as isotopic labeling
1153 Flavonoids Resveratrols Gallic Acids and Further Polyphenolics We have previously discussed the polyketide origin of some phenolic compounds based on the (C
2)
n motif Other polyphenols like gallic acids are directly
derived by the aromatization of shikimic acid (C6C
1 building
block Scheme 12) [13] The C6C
3 building blocks are availshy
able from the conversion of phenylalanine and tyrosine into cinnamic and p‐coumaric acids respectively and then by further hydroxylation steps (Scheme 15) These can dimerize into lignans (eg podophyllotoxin) [14 15] through radical processes or converted to low molecular weight compounds like eugenol coumarins or vanillin [16] The coenzyme A thioesters of these C
6C
3 acids can be used as initiator units
by specialized ketosynthases for an elongation by two acetyl units leading to aromatic polyketides like styrylpyrones or diarylheptanoids (eg curcumin) [17] Important comshypounds from this metabolism are flavonoids (C
6C
3C
6) [18]
and stilbenoids (C6C
2C
6) (a decarboxylation occurs during
the aryl cyclization) [19] which are synthesized by chalcone synthase and stilbene synthase respectively Flavonoids (eg catechin) and stilbenes (eg resveratrol) are present in large amounts in fruits and vegetables and may exert their radical scavenging properties in vivo
1154 Alkaloids Alkaloids are nitrogen‐containing compounds The nitrogen(s) can be involved in an amine function conferring basicity to the natural product (like ldquoalkalirdquo) or in less or nonbasic functions such as an amide a nitrile an isonitrile or an ammonium salt (quaternary amines) For amines protonation often occurs at physiological pH and may condition their biological activity In many cases the nitrogen is biogenetically derived from an amino acid We will thus discuss alkaloids according to their amino acid origin
Alkaloids Derived from the Krebs Cycle (Lysine and Ornithine Derived) As shown previously (Scheme 12) the Krebs cycle is a crucial metabolic process which leads to α‐ketoacids (oxaloacetic and 2‐oxoglutaric acids) Their enzymatic transamination affords the two amino acidsmdashaspartic acid and glutamic acidmdashwhich are the direct biosynthetic precursors of amino acids lysine and ornithine respectively These in turn produce cadaverine a ldquoC
5Nrdquo unit
and putrescine a ldquoC4Nrdquo unit which are major components
for the biosynthesis of important alkaloids as will be discussed later (Scheme 16) Additionally ornithine is a precursor of arginine another important amino acid
ornithine‐derived alkaloids (incorporating the c4n
unit) Putrescine is derived from the decarboxylation of ornithine and is a precursor of linear polyamines like spermshyine After enzymatic methylation of one amine of putrescine
C10 C15
C15
C10
C10
C30
CO2H
Chrysanthemic acid
OH
Menthol
O
HO
H
HOGlc
MeO2CLoganin (iridoid)
H
H
H
HH
OO
OParthenolide
O
O
OHOHO
OAcHO
OnC3H7
O
O
O
nC7H15
Thapsigargin
O
O
OO
H
H
O
Cleavage
H
H
Artemisinin
C20 C40
O
O
OO
OO
OHOHH
Bilobalide(highly rearrangedand missing 5 C)
CleavageH
Highoxidativecleavages
HO OAcH
AcO O OH
OOOBz
O
Ph
BzNH
OHFrom PHE Paclitaxel
C25O
H
HO
OHCH
OH Ophiobolin A
H
HO
H
H
HHCholesterol
(missing 3 C WM)
H
H H H
H
H
Hopene
β-Carotene
C30 - 3
C15
C20 - 5WM
WM
FIGURE 13 Chemical diversity in the terpene series (Wm Wagnerndashmeerwein shifts lost carbons bold bonds are remnant of primary building blocks)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 7
in the presence of S‐adenosylmethionine transamination of the other affords γ‐(N‐methylamino)aldehyde [20] The resulting cyclic iminium is a key intermediate in the formation of many medicinally important alkaloids such as
the plant‐derived compounds cocaine atropine or the calysshytegines [21 22] Indeed this iminium is a mannich acceptor which can react with various nucleophiles the first of those being the carbanion of acetyl‐CoA Thus after a stepwise
CO2H
RCinnamic acid (R=H)p-Coumaric acid (R=OH)
RO
PHETYR
OH
[H] [O]
lignans
OH
O
O
O
O
OMeMeO
OMe
C mdash C andor C mdash Oradical couplings
After E Zisomerization
for example Podophyllotoxin
O ORO
Coumarins(eg scopoletin)
[O]
Prenylationcyclizationcleavage[O]
O OO
Furocoumarins(eg psoralen)
RO
nAcetyl-CoA
thencyclization
n = 2 styrylpyrones
O O
OMe
n = 3 avonoids(from chalcone synthase)
C6C3
H
H H
From AcCoA
O
n = 3 stilbenoids(from stilbene synthase)
HO
OH
OHFrom AcCoA From AcCoA(ndashCO2)
Resveratrol
HO
OH
OH
OH
OH
H
Catechin
MeO Yangonin
From AcCoA
SCHEME 15 The phenylpropanoid biosynthetic pathways
LYS
CO2
Cadaverine
O NH
H2O
NH
Piperideineiminium
Pelletierine
AcAcCoA
CO2
NH
O
N
O
Pseudopelletierine
NH
Ph
OH
Ph
O
Lobeline
NH
δ+
δndash
N
H
H
N
H
OH
Lupinine
N
H N
H
(+)-Sparteine
N
NH
O (ndash)-Cytisine
NH
CO2H
Pipecolic acid
N
OHHO
OH
HO
H
Castanos permine
ORN
CO2
NH2 NH2 NH2 NH2
NH2
NH2
NH2
PutrescineO
NHMe
N-Methylpyrrolinium
2 timesAcCoA
NMe
ARG
n = 1 spermidinen = 2 homospermidine
NMe
O
HygrineCO2
CO2
MeN
O
[O]
TropinoneCO2
HNHO
OHOH
OH
Calystegine B2
MeN
OAtropine
O
Ph
HO
NMe
NMe
NMe
O
Cuscohygrine
HN
HN
NH2
( )n
O
n = 2
N
HHO
Retronecine
HH
H
H
H
HO
H H
H
SCHEME 16 Lysine‐ and ornithine‐derived alkaloid biosynthetic pathways (mind the structural similarities)
8 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
elongation by two AcCoA units either decarboxylation can occur leading to the acetonylpyrrolidine hygrine or a secshyond mannich reaction by the intramolecular attack of the acetoacetate anion onto an oxidation‐derived pyrrolinium leading to the tropane skeleton (tropinone) The acetoacetyl‐CoA intermediate can also react intermolecularly with another pyrrolinium cation leading to cuscohygrine after decarboxylation Finally the pyrrolizidine alkaloids [23] are derived from homospermidine which when submitted to terminal oxidative deamination leads to the bicyclic skelshyeton of retronecine and further Senecio alkaloids We can mention herein that ornithine is a biosynthetic precursor of arginine bearing a guanidine function which is an intermediate toward the toxic compounds tetrodotoxin and saxitoxin (not shown)
lysine‐derived alkaloids (incorporating the c5n
unit) From lysine to piperidine alkaloids the biosynthetic steps parallel the one previously described from ornithine Indeed the oxidative deamination of cadaverine affords a δ‐amino aldehyde which cyclizes through imine formation into piperideine Protonation results in a mannich acceptor which is able to react with various nucleophiles such as β‐ketothioester anions The first product of these reactions is pelletierine which can further react through an intramolecular mannich
reaction leading to pseudopelletierine Quinolizidines [24] can also be formed first from the mannich reaction of the piperideine acceptor with the corresponding enamine nucleoshyphile and then after additional transformation steps leading for example to lupinine sparteine or cytisine
Indolizidine alkaloids [15] such as castanospermine and swainsonine are formed from pipecolic acid an amino acid derived from lysine which can be elongated by malonyl‐CoA followed by ring closure When protonated these alkaloids are oxonium mimics strongly inhibiting glycosidases
Tyrosine‐ and Phenylalanine‐Derived Alkaloids Tyrosine and phenylalanine amino acids are bearing the phenylshyethylamine moiety of many medicinally relevant alkaloids Further hydroxylations on the aromatic carbocycle or on the aliphatic part can be observed methylations can occur on phenolic oxygens and on the amine leading to catecholamines (adrenaline noradrenaline dopamine) Arylethylamines are also usual to react with endogenous aldehydes through PictetndashSpengler reactions [25] leading to important biosynthetic intermediates (Scheme 17) like
bull Reticuline from the reaction with 4‐hydroxyphenylacetshyaldehyde toward benzyltetrahydroisoquinoline alkaloids
NH2
Phenylethylamines
RONH TetrahydroisoquinolinesRO
RʹCHO
Rʹ
NH
(CH2)n
MeO
HO
Benzyltetrahydroisoquinoline(n = 1) for example reticuline
orPhenethyltetrahydroisoquinoline
(n = 2) eg autumnaline
IntermolecularCmdashO phenol
couplings
Curarealkaloids
IntramolecularC mdash C phenol
couplings
ONMe
HHO
H
HO
Morphine
NMe
MeO
HO
MeOOH
Isoboldine
O
O
OMe
NO2
CO2H
Aristolochic acid
OMe
O
NHAcMeO
MeOMeO
Colchicine
N
O
O
OMe
OMeBerberine
IntramolecularCmdash C coupling
and furtheroxidative events
Rʹ derivedfrom PHE
Pictet-Spenglerreaction
TYR
HO
HO CHO
HNHO
HO
OH
OMeO
OH
NMe
[O]
GalantamineNorbelladine
Rʹ derived fromsecologanin
NAc
HO
HO
O
CO2MeH
H
H
OHIpecosideaglycone
Ipecac alkaloids
1ClostCmdash C cleavage
and ring extension
H
ROH
1C lost
Cleavage
SCHEME 17 Tyrosine‐derived alkaloid biosynthetic pathways (double head arrows figure bond cleavages during biosynthetic processes)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 9
morphine berberine tubocurarine isoboldine or the highly modified aristolochic acid [26 27]
bull Automnaline from the reaction with 3‐(4‐hydroxyphenyl)propanal toward phenylethyltetrahydroisoquinoline alkaloids colchicine cephalotaxine or schelhammerishycine [28 29]
bull Ipecoside from the reaction of dopamine with secoloshyganin toward terpene tetrahydroisoquinoline alkaloids ipecoside or emetine
Lastly norbelladine (top of Scheme 17) is issued from the reductive amination of 34‐dihydroxybenzaldehyde (derived from phenylalanine) with tyramine (derived from tyrosine) and constitutes a biosynthetic node leading to Amaryllidaceae alkaloids such as galantamine crinine or lycorine depending on the topology of phenolic couplings In all these biosynthetic routes radical phenolic couplings are key reactions for CC and CO bond formations and rearrangements [30 31]
Tryptophan‐Derived Indole and Indole Monoterpene Alkaloids As for alkaloids derived from tyrosine and phenylalanine those derived from tryptophan are formed after decarboxylation of the amino acid (into tryptamine) and possible hydroxylation of the aromatic carbocycle (eg serotonin) and N‐methylation (eg psilocin) As previously tryptamine can also react through PictetndashSpengler reactions to form tetrahydro‐β‐carbolines which can be aromatized for example into harmine (Scheme 18) [16]
When the aldehyde partner of the PictetndashSpengler reacshytion with tryptamine is the terpene secologanin strictosidine is formed as an entry toward the vast monoterpene indole alkaloids [32 33] Hydrolysis of the glucosidic part releases the strictosidine aglycone bearing an aldehyde while iminshyium formation and further cyclization and reduction can lead to ajmalicine (from oxocyclization) or yohimbine (from carshybocyclization) These alkaloids are referred to as from the Corynanthe type with the monoterpene carbon skeleton unmodified Although it misses one carbon and has a very
RO
Carbonylcompounds
TRPNH
NH2
Indolethylamines(eg serotonine)
RONH
NH
Rʹ NH
N
MeOPictet-Spenglerreaction for example Harmineβ-Carbolines
NH
NHH
O
MeO2C
MeO2C
OH
H
H
Rʹ derived from secologanin
Strictosidine aglycone
NH
N
OH
H
H
Ajmalicine
N
N
O
O
H
H
H
Strychnine (C lost)Corynanthe type
Aspidosperma type Iboga type
N
N
OH
OMe
Quinine (C lost)
N
NH CO2Me
Catharanthine
NH
N
CO2MeH
OAc
OH
Vindoline
Complextransformations
NH
NMe
HN
MeN
H
H
Chimonanthine
Cyclizationdimerization
Prenylationcyclization
NH
NMeHO2C
H
Lysergic acid
H
H
Ergot alkaloidsPyrroloindoles Indole monoterpene
1C lost
1C lostFrom AcCoA
SCHEME 18 Tryptophan‐derived alkaloid biosynthetic pathways (gray parts monoterpenic units)
10 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
different structure strychnine is related to the Corynanthe alkaloids incorporating two carbons from acetyl‐CoA Highly modified monoterpene skeletons are derived from the Corynanthe core through CC bond breaking and reorshyganization leading to Iboga‐type (eg catharanthine) and Aspidosperma‐type (eg vindoline) alkaloids The antishycancer drug vinblastine is a heterodimer resulting from the nucleophilic attack of vindoline on a mannich acceptor resulting from catharanthine found in madagascar perishywinkle (Catharanthus roseus) The heteroaromatic comshypounds ellipticine camptothecin and quinine are also derived from a Corynanthe‐type precursor although in this case the biosynthetic relationship may not be obvious due to deep modifications of the skeleton
Finally two important classes of compounds have to be mentioned since they have inspired many synthetic chemists The pyrroloindole alkaloids result from the cyclization of tryptamine as found in physostigmine (formed by a cationic mechanism after methylation in position 3 of the indole not shown) or in chimonanthine (presumably formed by a radical coupling mechanism Scheme 18) The ergot alkaloids are derived from the 33‐dimethylallylation on position 4 of the indole in trypshytophan whose further cyclization and oxidation processes afford the natural products (eg lysergic acid Scheme 18 and ergotamine) which have had important medical applications [34]
NRPS Metabolites and Peptides NRPS enzymes assemble amino acids including nonproteinogenic ones into oligopeptides The enzymes contain several modules and especially an adenylation domain (A) which specifically selects and activates the amino acid to be transferred as a thioester on the nearby peptidyl carrier protein (PCP) [2] A condensation module (C) then catalyzes the formation of the peptide bonds between the newly introduced amino acyl‐PCP (bearing a free amine) and the elongated peptidyl‐PCP thioester At the end of the elongation a cyclization can occur into cyclopeptides but the peptide can also be
transferred to auxiliary enzymes like methyltransferases glycosyltransferases or oxidases (vancomycins are typical products of such functionalizations) [35 36] The formation of heterocycles is also frequently encountered in this metabolism as in penicillins that are derived from the tripeptide α‐aminoadipoyl‐cysteinyl‐valine or telomestatin (Fig 14) [2]
Other Alkaloid Origins There are many other nitrogen sources involved in alkaloid biosyntheses for example nicotinic acid (originated from aspartic acid and intershymediate in nicotine and anabasine biosyntheses) and anthranilic acid (originated from tryptophan and intermedishyate toward acridines or aurachins) The amination reaction (eg through transamination of carbonyl compounds) is also a way to introduce nitrogens in natural products for example from fatty acids steroids (toward Solanum alkaloids or cyclopamines) or other terpenoids (aconitine and atisine have diterpene skeletons while Daphniphyllum alkaloids are triterpene derivatives) Finally nucleic acids can also be precursors of alkaloids like the well‐known caffeine
12 FROM BIOSYNTHESIS TO TOTAL SYNTHESIS STRATEGIES TOWARD THE NATURAL PRODUCT CHEMICAL SPACE
121 The Chemical Space of Natural Products
Natural products occupy an important place in human com munities as demonstrated by their vast use from ancient times to nowadays like dyes fibers oils pershyfumes agrochemicals or drugs Broadly both primary and secondary metabolites could be classified as natural products while the latter as discussed previously are usushyally regarded as the ldquonatural productsrdquo owing to their comshyplexity and diversity arising from a variety of biosynthetic pathways The structural chemical diversity found among
N
S
CO2HO
HNPhH2C
O
Penicillin G
OH
HN
HN
O
O
ONH2
O
OHO
NHMe
OCl
O
NH
NH
NH
O
ClHO
O
HN
H
H
HOOH
OHHO2C
Vancomycin aglycone
ON
NO
O
NN
O
O
N
N O
Me
S
N N
OMeH
Telomestatin
L-AAA-L-CYS-D-VAL
Enzymes
FIGURE 14 Structural diversity of nonribosomal peptide compounds (AAA α‐aminoadipic acid)
FROm BIOSyNTHESIS TO TOTAL SyNTHESIS STRATEGIES TOWARD THE NATuRAL PRODuCT CHEmICAL SPACE 11
all living organisms defining the chemical space of natural products [37] is the consequence of their evolution occurshyring as an adaptation of organisms to their environment It is commonly believed that secondary metabolites are proshyduced as messengers by living organisms or as weapons against enemies and thus they should have certain biological activities in a medicinal point of view [38] Indeed natural products are regarded as one of the main sources of medicines (Fig 15) From the traditional medicinal extracts to every single bioactive molecule the methods of extraction purification identification and biological investigation of natural products have been well established Their complex structures and interesting properties have attracted synthetic chemists to accomplish their total syntheses and that of medicinally relevant anashylogs sometimes in the industrial context [39] Thus tarshygeting the chemical space of natural products has never been more relevant than today Although the discovery of natural products demands time and labor‐consuming manipulations it is worth to notice that the knowledge on this chemical space is still continually growing while biological advances allow for discovering and undershystanding potential targets However increasing the chemical space of human‐made compounds based on natural products should benefit from transdisciplinary colshylaborations such as the use of coupled biosynthetic and chemical synthetic methods to design original ldquounnatural natural productsrdquo [40]
122 The Biosynthetic Pathways as an Inspiration for Synthetic Challenges
1221 Precursor‐Directed Biosyntheses and Mutashysynthetic Strategies to Increase the Chemical Space of Natural Products As the genetics and biochemistry of natural product biosynthesis are better understood novel biosynthetic techniques have been developed to study and generate new diversity in natural product analogs (Scheme 19) Precursor‐directed biosynthesis (PDB) is considered as the earliest example of combining chemical and biological methods for the generation of complex natural product analogs [41 42] This approach compared with the biosynthetic pathway of wild‐type metabolites (Scheme 19a) involves the feeding of analogs of the natural biosynthetic building blocks to the living organisms (Scheme 19b) usually bacteria or fungi which incorporate the modified precursors into the biosynthesized compound mutasynthesis also termed as mutational biosynthesis (mBS) involves the inactivation of a key step of the bioshysynthesis in a mutant microorganism (Scheme 19c) which can then be fed by various modified or advanced building blocks (mutasynthons Scheme 19d) [43] These mutasshyynthons could not be incorporated by the wild type due to specificities of the enzymatic machinery Build up on PDB mBS eliminates the natural biosynthetic intermediate thus generating a less complex mixture of metabolites and making the purification or yield of target products better
O NH
OHO
O
OO
O
O
OH
HOO
O O
OH
HOO
NH ONH2
O
ONH
NH
O
OCl Cl
O
O
OH
HN
HN
HN
HN
OO
HO
HN
OH
OHOH
O
O
HO
HO
OHO
O
OHH2N
O
OH3C
H
O
H
CH3
CH3
H
O O
NO
H
O
HO
O
NO
O
NH
HN
OHO
HONH
ON
H
HO
O
H2N
O OH
ONH
OH
ON OHO
OH
HO OS OH
OOOH
OHO
O
ON
OO
O
HO
O
O OH
OO
Micafunginantifungal
Rapamycinimmunosuppressive
Galantamineanti-Alzheimer
Taxolanticancer
Artemisininantimalarial
Vancomycinantibiotic
FIGURE 15 Some famous natural products currently used as drugs
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 5
farnesylfarnesyl (C30
triterpene precursor) diphosphates can also be obtained by further additions of IPP leading to longer linear intermediates
The cyclization of linear precursors is achieved by speshycialized cyclases which generate a poorly functionalized natural product framework [10 11] Auxiliary enzymes such as oxygenases then increase the complexity and the diversity
of compounds by further functionalization (Scheme 13b) [12] A high degree of oxidation can be observed in comshypounds like thapsigargin paclitaxel or bilobalide (Fig 13) The biosynthesis of this last compound for example involves a high oxygenation pattern two Wagnerndashmeerwein rearrangements and several oxidative cleavages leading to the loss of five carbons The resulting natural products can
KS
S-ACP
O O
KR
S-ACP
OH O
S-MAT
O
YesNo
R
R
R
YesNoDH
S-ACP
OR
YesNoER
S-ACP
OR
YesNoTE
OH
OR
New cycle
New cycleor
release
New cycleor
release
New cycleor
release
Release
R in
crem
ente
d by
two
carb
ons
HO
O
S-ACP
O
Claisen condensation (ndashCO2)
reduction (NADPH)
dehydration (ndashH2O)
+
reduction (NADPH)
hydrolysis (H2O)
O O NH
OH
OH
H H
H H
Hirsutellone B(mixed PKSNRPS
product)
OO
O
OHOH
HO OH
Norsolorinic acid
O
O
O
O CHO
OH
HH
H
H
HH
OHHemibrevetoxin B
OOHO
OHO
HOHO OH
Erythronolide A
OH
OH
HO
Panaxytriol
From ahexanoylstartingblock
H
H
Fromtyrosine
H
1C lost fromdecarboxylation
FIGURE 12 Chemical logic of polyketide construction leading to variable functionalization of the elongated acyl chain and examples of resulting chemical diversity ACP acyl carrier protein DH dehydratase ER enoyl reductase KR ketoreductase KS ketosynthase mAT malonyl acyl transferase TE thioesterase
DMAPPOP2O6
3ndashOP2O6
3ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
OP2O63ndash
Geranyl diphosphate (GPP)2 times IPP
H H
Geranylgeranyl diphosphate (GGPP)
Examplesverticillane
casbanetaxane
phorbol
Exampleslabdane
pimaranekauraneabietane
aphidicolanegibberellane
IPP
SCHEME 14 Early assembly of C5 units in terpene biosynthesis leading to diterpenes (C
20)
6 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
thus be extremely modified with structures whose biogenetic origin is far from being obvious at first sight and cannot be determined without further experiments such as isotopic labeling
1153 Flavonoids Resveratrols Gallic Acids and Further Polyphenolics We have previously discussed the polyketide origin of some phenolic compounds based on the (C
2)
n motif Other polyphenols like gallic acids are directly
derived by the aromatization of shikimic acid (C6C
1 building
block Scheme 12) [13] The C6C
3 building blocks are availshy
able from the conversion of phenylalanine and tyrosine into cinnamic and p‐coumaric acids respectively and then by further hydroxylation steps (Scheme 15) These can dimerize into lignans (eg podophyllotoxin) [14 15] through radical processes or converted to low molecular weight compounds like eugenol coumarins or vanillin [16] The coenzyme A thioesters of these C
6C
3 acids can be used as initiator units
by specialized ketosynthases for an elongation by two acetyl units leading to aromatic polyketides like styrylpyrones or diarylheptanoids (eg curcumin) [17] Important comshypounds from this metabolism are flavonoids (C
6C
3C
6) [18]
and stilbenoids (C6C
2C
6) (a decarboxylation occurs during
the aryl cyclization) [19] which are synthesized by chalcone synthase and stilbene synthase respectively Flavonoids (eg catechin) and stilbenes (eg resveratrol) are present in large amounts in fruits and vegetables and may exert their radical scavenging properties in vivo
1154 Alkaloids Alkaloids are nitrogen‐containing compounds The nitrogen(s) can be involved in an amine function conferring basicity to the natural product (like ldquoalkalirdquo) or in less or nonbasic functions such as an amide a nitrile an isonitrile or an ammonium salt (quaternary amines) For amines protonation often occurs at physiological pH and may condition their biological activity In many cases the nitrogen is biogenetically derived from an amino acid We will thus discuss alkaloids according to their amino acid origin
Alkaloids Derived from the Krebs Cycle (Lysine and Ornithine Derived) As shown previously (Scheme 12) the Krebs cycle is a crucial metabolic process which leads to α‐ketoacids (oxaloacetic and 2‐oxoglutaric acids) Their enzymatic transamination affords the two amino acidsmdashaspartic acid and glutamic acidmdashwhich are the direct biosynthetic precursors of amino acids lysine and ornithine respectively These in turn produce cadaverine a ldquoC
5Nrdquo unit
and putrescine a ldquoC4Nrdquo unit which are major components
for the biosynthesis of important alkaloids as will be discussed later (Scheme 16) Additionally ornithine is a precursor of arginine another important amino acid
ornithine‐derived alkaloids (incorporating the c4n
unit) Putrescine is derived from the decarboxylation of ornithine and is a precursor of linear polyamines like spermshyine After enzymatic methylation of one amine of putrescine
C10 C15
C15
C10
C10
C30
CO2H
Chrysanthemic acid
OH
Menthol
O
HO
H
HOGlc
MeO2CLoganin (iridoid)
H
H
H
HH
OO
OParthenolide
O
O
OHOHO
OAcHO
OnC3H7
O
O
O
nC7H15
Thapsigargin
O
O
OO
H
H
O
Cleavage
H
H
Artemisinin
C20 C40
O
O
OO
OO
OHOHH
Bilobalide(highly rearrangedand missing 5 C)
CleavageH
Highoxidativecleavages
HO OAcH
AcO O OH
OOOBz
O
Ph
BzNH
OHFrom PHE Paclitaxel
C25O
H
HO
OHCH
OH Ophiobolin A
H
HO
H
H
HHCholesterol
(missing 3 C WM)
H
H H H
H
H
Hopene
β-Carotene
C30 - 3
C15
C20 - 5WM
WM
FIGURE 13 Chemical diversity in the terpene series (Wm Wagnerndashmeerwein shifts lost carbons bold bonds are remnant of primary building blocks)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 7
in the presence of S‐adenosylmethionine transamination of the other affords γ‐(N‐methylamino)aldehyde [20] The resulting cyclic iminium is a key intermediate in the formation of many medicinally important alkaloids such as
the plant‐derived compounds cocaine atropine or the calysshytegines [21 22] Indeed this iminium is a mannich acceptor which can react with various nucleophiles the first of those being the carbanion of acetyl‐CoA Thus after a stepwise
CO2H
RCinnamic acid (R=H)p-Coumaric acid (R=OH)
RO
PHETYR
OH
[H] [O]
lignans
OH
O
O
O
O
OMeMeO
OMe
C mdash C andor C mdash Oradical couplings
After E Zisomerization
for example Podophyllotoxin
O ORO
Coumarins(eg scopoletin)
[O]
Prenylationcyclizationcleavage[O]
O OO
Furocoumarins(eg psoralen)
RO
nAcetyl-CoA
thencyclization
n = 2 styrylpyrones
O O
OMe
n = 3 avonoids(from chalcone synthase)
C6C3
H
H H
From AcCoA
O
n = 3 stilbenoids(from stilbene synthase)
HO
OH
OHFrom AcCoA From AcCoA(ndashCO2)
Resveratrol
HO
OH
OH
OH
OH
H
Catechin
MeO Yangonin
From AcCoA
SCHEME 15 The phenylpropanoid biosynthetic pathways
LYS
CO2
Cadaverine
O NH
H2O
NH
Piperideineiminium
Pelletierine
AcAcCoA
CO2
NH
O
N
O
Pseudopelletierine
NH
Ph
OH
Ph
O
Lobeline
NH
δ+
δndash
N
H
H
N
H
OH
Lupinine
N
H N
H
(+)-Sparteine
N
NH
O (ndash)-Cytisine
NH
CO2H
Pipecolic acid
N
OHHO
OH
HO
H
Castanos permine
ORN
CO2
NH2 NH2 NH2 NH2
NH2
NH2
NH2
PutrescineO
NHMe
N-Methylpyrrolinium
2 timesAcCoA
NMe
ARG
n = 1 spermidinen = 2 homospermidine
NMe
O
HygrineCO2
CO2
MeN
O
[O]
TropinoneCO2
HNHO
OHOH
OH
Calystegine B2
MeN
OAtropine
O
Ph
HO
NMe
NMe
NMe
O
Cuscohygrine
HN
HN
NH2
( )n
O
n = 2
N
HHO
Retronecine
HH
H
H
H
HO
H H
H
SCHEME 16 Lysine‐ and ornithine‐derived alkaloid biosynthetic pathways (mind the structural similarities)
8 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
elongation by two AcCoA units either decarboxylation can occur leading to the acetonylpyrrolidine hygrine or a secshyond mannich reaction by the intramolecular attack of the acetoacetate anion onto an oxidation‐derived pyrrolinium leading to the tropane skeleton (tropinone) The acetoacetyl‐CoA intermediate can also react intermolecularly with another pyrrolinium cation leading to cuscohygrine after decarboxylation Finally the pyrrolizidine alkaloids [23] are derived from homospermidine which when submitted to terminal oxidative deamination leads to the bicyclic skelshyeton of retronecine and further Senecio alkaloids We can mention herein that ornithine is a biosynthetic precursor of arginine bearing a guanidine function which is an intermediate toward the toxic compounds tetrodotoxin and saxitoxin (not shown)
lysine‐derived alkaloids (incorporating the c5n
unit) From lysine to piperidine alkaloids the biosynthetic steps parallel the one previously described from ornithine Indeed the oxidative deamination of cadaverine affords a δ‐amino aldehyde which cyclizes through imine formation into piperideine Protonation results in a mannich acceptor which is able to react with various nucleophiles such as β‐ketothioester anions The first product of these reactions is pelletierine which can further react through an intramolecular mannich
reaction leading to pseudopelletierine Quinolizidines [24] can also be formed first from the mannich reaction of the piperideine acceptor with the corresponding enamine nucleoshyphile and then after additional transformation steps leading for example to lupinine sparteine or cytisine
Indolizidine alkaloids [15] such as castanospermine and swainsonine are formed from pipecolic acid an amino acid derived from lysine which can be elongated by malonyl‐CoA followed by ring closure When protonated these alkaloids are oxonium mimics strongly inhibiting glycosidases
Tyrosine‐ and Phenylalanine‐Derived Alkaloids Tyrosine and phenylalanine amino acids are bearing the phenylshyethylamine moiety of many medicinally relevant alkaloids Further hydroxylations on the aromatic carbocycle or on the aliphatic part can be observed methylations can occur on phenolic oxygens and on the amine leading to catecholamines (adrenaline noradrenaline dopamine) Arylethylamines are also usual to react with endogenous aldehydes through PictetndashSpengler reactions [25] leading to important biosynthetic intermediates (Scheme 17) like
bull Reticuline from the reaction with 4‐hydroxyphenylacetshyaldehyde toward benzyltetrahydroisoquinoline alkaloids
NH2
Phenylethylamines
RONH TetrahydroisoquinolinesRO
RʹCHO
Rʹ
NH
(CH2)n
MeO
HO
Benzyltetrahydroisoquinoline(n = 1) for example reticuline
orPhenethyltetrahydroisoquinoline
(n = 2) eg autumnaline
IntermolecularCmdashO phenol
couplings
Curarealkaloids
IntramolecularC mdash C phenol
couplings
ONMe
HHO
H
HO
Morphine
NMe
MeO
HO
MeOOH
Isoboldine
O
O
OMe
NO2
CO2H
Aristolochic acid
OMe
O
NHAcMeO
MeOMeO
Colchicine
N
O
O
OMe
OMeBerberine
IntramolecularCmdash C coupling
and furtheroxidative events
Rʹ derivedfrom PHE
Pictet-Spenglerreaction
TYR
HO
HO CHO
HNHO
HO
OH
OMeO
OH
NMe
[O]
GalantamineNorbelladine
Rʹ derived fromsecologanin
NAc
HO
HO
O
CO2MeH
H
H
OHIpecosideaglycone
Ipecac alkaloids
1ClostCmdash C cleavage
and ring extension
H
ROH
1C lost
Cleavage
SCHEME 17 Tyrosine‐derived alkaloid biosynthetic pathways (double head arrows figure bond cleavages during biosynthetic processes)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 9
morphine berberine tubocurarine isoboldine or the highly modified aristolochic acid [26 27]
bull Automnaline from the reaction with 3‐(4‐hydroxyphenyl)propanal toward phenylethyltetrahydroisoquinoline alkaloids colchicine cephalotaxine or schelhammerishycine [28 29]
bull Ipecoside from the reaction of dopamine with secoloshyganin toward terpene tetrahydroisoquinoline alkaloids ipecoside or emetine
Lastly norbelladine (top of Scheme 17) is issued from the reductive amination of 34‐dihydroxybenzaldehyde (derived from phenylalanine) with tyramine (derived from tyrosine) and constitutes a biosynthetic node leading to Amaryllidaceae alkaloids such as galantamine crinine or lycorine depending on the topology of phenolic couplings In all these biosynthetic routes radical phenolic couplings are key reactions for CC and CO bond formations and rearrangements [30 31]
Tryptophan‐Derived Indole and Indole Monoterpene Alkaloids As for alkaloids derived from tyrosine and phenylalanine those derived from tryptophan are formed after decarboxylation of the amino acid (into tryptamine) and possible hydroxylation of the aromatic carbocycle (eg serotonin) and N‐methylation (eg psilocin) As previously tryptamine can also react through PictetndashSpengler reactions to form tetrahydro‐β‐carbolines which can be aromatized for example into harmine (Scheme 18) [16]
When the aldehyde partner of the PictetndashSpengler reacshytion with tryptamine is the terpene secologanin strictosidine is formed as an entry toward the vast monoterpene indole alkaloids [32 33] Hydrolysis of the glucosidic part releases the strictosidine aglycone bearing an aldehyde while iminshyium formation and further cyclization and reduction can lead to ajmalicine (from oxocyclization) or yohimbine (from carshybocyclization) These alkaloids are referred to as from the Corynanthe type with the monoterpene carbon skeleton unmodified Although it misses one carbon and has a very
RO
Carbonylcompounds
TRPNH
NH2
Indolethylamines(eg serotonine)
RONH
NH
Rʹ NH
N
MeOPictet-Spenglerreaction for example Harmineβ-Carbolines
NH
NHH
O
MeO2C
MeO2C
OH
H
H
Rʹ derived from secologanin
Strictosidine aglycone
NH
N
OH
H
H
Ajmalicine
N
N
O
O
H
H
H
Strychnine (C lost)Corynanthe type
Aspidosperma type Iboga type
N
N
OH
OMe
Quinine (C lost)
N
NH CO2Me
Catharanthine
NH
N
CO2MeH
OAc
OH
Vindoline
Complextransformations
NH
NMe
HN
MeN
H
H
Chimonanthine
Cyclizationdimerization
Prenylationcyclization
NH
NMeHO2C
H
Lysergic acid
H
H
Ergot alkaloidsPyrroloindoles Indole monoterpene
1C lost
1C lostFrom AcCoA
SCHEME 18 Tryptophan‐derived alkaloid biosynthetic pathways (gray parts monoterpenic units)
10 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
different structure strychnine is related to the Corynanthe alkaloids incorporating two carbons from acetyl‐CoA Highly modified monoterpene skeletons are derived from the Corynanthe core through CC bond breaking and reorshyganization leading to Iboga‐type (eg catharanthine) and Aspidosperma‐type (eg vindoline) alkaloids The antishycancer drug vinblastine is a heterodimer resulting from the nucleophilic attack of vindoline on a mannich acceptor resulting from catharanthine found in madagascar perishywinkle (Catharanthus roseus) The heteroaromatic comshypounds ellipticine camptothecin and quinine are also derived from a Corynanthe‐type precursor although in this case the biosynthetic relationship may not be obvious due to deep modifications of the skeleton
Finally two important classes of compounds have to be mentioned since they have inspired many synthetic chemists The pyrroloindole alkaloids result from the cyclization of tryptamine as found in physostigmine (formed by a cationic mechanism after methylation in position 3 of the indole not shown) or in chimonanthine (presumably formed by a radical coupling mechanism Scheme 18) The ergot alkaloids are derived from the 33‐dimethylallylation on position 4 of the indole in trypshytophan whose further cyclization and oxidation processes afford the natural products (eg lysergic acid Scheme 18 and ergotamine) which have had important medical applications [34]
NRPS Metabolites and Peptides NRPS enzymes assemble amino acids including nonproteinogenic ones into oligopeptides The enzymes contain several modules and especially an adenylation domain (A) which specifically selects and activates the amino acid to be transferred as a thioester on the nearby peptidyl carrier protein (PCP) [2] A condensation module (C) then catalyzes the formation of the peptide bonds between the newly introduced amino acyl‐PCP (bearing a free amine) and the elongated peptidyl‐PCP thioester At the end of the elongation a cyclization can occur into cyclopeptides but the peptide can also be
transferred to auxiliary enzymes like methyltransferases glycosyltransferases or oxidases (vancomycins are typical products of such functionalizations) [35 36] The formation of heterocycles is also frequently encountered in this metabolism as in penicillins that are derived from the tripeptide α‐aminoadipoyl‐cysteinyl‐valine or telomestatin (Fig 14) [2]
Other Alkaloid Origins There are many other nitrogen sources involved in alkaloid biosyntheses for example nicotinic acid (originated from aspartic acid and intershymediate in nicotine and anabasine biosyntheses) and anthranilic acid (originated from tryptophan and intermedishyate toward acridines or aurachins) The amination reaction (eg through transamination of carbonyl compounds) is also a way to introduce nitrogens in natural products for example from fatty acids steroids (toward Solanum alkaloids or cyclopamines) or other terpenoids (aconitine and atisine have diterpene skeletons while Daphniphyllum alkaloids are triterpene derivatives) Finally nucleic acids can also be precursors of alkaloids like the well‐known caffeine
12 FROM BIOSYNTHESIS TO TOTAL SYNTHESIS STRATEGIES TOWARD THE NATURAL PRODUCT CHEMICAL SPACE
121 The Chemical Space of Natural Products
Natural products occupy an important place in human com munities as demonstrated by their vast use from ancient times to nowadays like dyes fibers oils pershyfumes agrochemicals or drugs Broadly both primary and secondary metabolites could be classified as natural products while the latter as discussed previously are usushyally regarded as the ldquonatural productsrdquo owing to their comshyplexity and diversity arising from a variety of biosynthetic pathways The structural chemical diversity found among
N
S
CO2HO
HNPhH2C
O
Penicillin G
OH
HN
HN
O
O
ONH2
O
OHO
NHMe
OCl
O
NH
NH
NH
O
ClHO
O
HN
H
H
HOOH
OHHO2C
Vancomycin aglycone
ON
NO
O
NN
O
O
N
N O
Me
S
N N
OMeH
Telomestatin
L-AAA-L-CYS-D-VAL
Enzymes
FIGURE 14 Structural diversity of nonribosomal peptide compounds (AAA α‐aminoadipic acid)
FROm BIOSyNTHESIS TO TOTAL SyNTHESIS STRATEGIES TOWARD THE NATuRAL PRODuCT CHEmICAL SPACE 11
all living organisms defining the chemical space of natural products [37] is the consequence of their evolution occurshyring as an adaptation of organisms to their environment It is commonly believed that secondary metabolites are proshyduced as messengers by living organisms or as weapons against enemies and thus they should have certain biological activities in a medicinal point of view [38] Indeed natural products are regarded as one of the main sources of medicines (Fig 15) From the traditional medicinal extracts to every single bioactive molecule the methods of extraction purification identification and biological investigation of natural products have been well established Their complex structures and interesting properties have attracted synthetic chemists to accomplish their total syntheses and that of medicinally relevant anashylogs sometimes in the industrial context [39] Thus tarshygeting the chemical space of natural products has never been more relevant than today Although the discovery of natural products demands time and labor‐consuming manipulations it is worth to notice that the knowledge on this chemical space is still continually growing while biological advances allow for discovering and undershystanding potential targets However increasing the chemical space of human‐made compounds based on natural products should benefit from transdisciplinary colshylaborations such as the use of coupled biosynthetic and chemical synthetic methods to design original ldquounnatural natural productsrdquo [40]
122 The Biosynthetic Pathways as an Inspiration for Synthetic Challenges
1221 Precursor‐Directed Biosyntheses and Mutashysynthetic Strategies to Increase the Chemical Space of Natural Products As the genetics and biochemistry of natural product biosynthesis are better understood novel biosynthetic techniques have been developed to study and generate new diversity in natural product analogs (Scheme 19) Precursor‐directed biosynthesis (PDB) is considered as the earliest example of combining chemical and biological methods for the generation of complex natural product analogs [41 42] This approach compared with the biosynthetic pathway of wild‐type metabolites (Scheme 19a) involves the feeding of analogs of the natural biosynthetic building blocks to the living organisms (Scheme 19b) usually bacteria or fungi which incorporate the modified precursors into the biosynthesized compound mutasynthesis also termed as mutational biosynthesis (mBS) involves the inactivation of a key step of the bioshysynthesis in a mutant microorganism (Scheme 19c) which can then be fed by various modified or advanced building blocks (mutasynthons Scheme 19d) [43] These mutasshyynthons could not be incorporated by the wild type due to specificities of the enzymatic machinery Build up on PDB mBS eliminates the natural biosynthetic intermediate thus generating a less complex mixture of metabolites and making the purification or yield of target products better
O NH
OHO
O
OO
O
O
OH
HOO
O O
OH
HOO
NH ONH2
O
ONH
NH
O
OCl Cl
O
O
OH
HN
HN
HN
HN
OO
HO
HN
OH
OHOH
O
O
HO
HO
OHO
O
OHH2N
O
OH3C
H
O
H
CH3
CH3
H
O O
NO
H
O
HO
O
NO
O
NH
HN
OHO
HONH
ON
H
HO
O
H2N
O OH
ONH
OH
ON OHO
OH
HO OS OH
OOOH
OHO
O
ON
OO
O
HO
O
O OH
OO
Micafunginantifungal
Rapamycinimmunosuppressive
Galantamineanti-Alzheimer
Taxolanticancer
Artemisininantimalarial
Vancomycinantibiotic
FIGURE 15 Some famous natural products currently used as drugs
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product
6 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
thus be extremely modified with structures whose biogenetic origin is far from being obvious at first sight and cannot be determined without further experiments such as isotopic labeling
1153 Flavonoids Resveratrols Gallic Acids and Further Polyphenolics We have previously discussed the polyketide origin of some phenolic compounds based on the (C
2)
n motif Other polyphenols like gallic acids are directly
derived by the aromatization of shikimic acid (C6C
1 building
block Scheme 12) [13] The C6C
3 building blocks are availshy
able from the conversion of phenylalanine and tyrosine into cinnamic and p‐coumaric acids respectively and then by further hydroxylation steps (Scheme 15) These can dimerize into lignans (eg podophyllotoxin) [14 15] through radical processes or converted to low molecular weight compounds like eugenol coumarins or vanillin [16] The coenzyme A thioesters of these C
6C
3 acids can be used as initiator units
by specialized ketosynthases for an elongation by two acetyl units leading to aromatic polyketides like styrylpyrones or diarylheptanoids (eg curcumin) [17] Important comshypounds from this metabolism are flavonoids (C
6C
3C
6) [18]
and stilbenoids (C6C
2C
6) (a decarboxylation occurs during
the aryl cyclization) [19] which are synthesized by chalcone synthase and stilbene synthase respectively Flavonoids (eg catechin) and stilbenes (eg resveratrol) are present in large amounts in fruits and vegetables and may exert their radical scavenging properties in vivo
1154 Alkaloids Alkaloids are nitrogen‐containing compounds The nitrogen(s) can be involved in an amine function conferring basicity to the natural product (like ldquoalkalirdquo) or in less or nonbasic functions such as an amide a nitrile an isonitrile or an ammonium salt (quaternary amines) For amines protonation often occurs at physiological pH and may condition their biological activity In many cases the nitrogen is biogenetically derived from an amino acid We will thus discuss alkaloids according to their amino acid origin
Alkaloids Derived from the Krebs Cycle (Lysine and Ornithine Derived) As shown previously (Scheme 12) the Krebs cycle is a crucial metabolic process which leads to α‐ketoacids (oxaloacetic and 2‐oxoglutaric acids) Their enzymatic transamination affords the two amino acidsmdashaspartic acid and glutamic acidmdashwhich are the direct biosynthetic precursors of amino acids lysine and ornithine respectively These in turn produce cadaverine a ldquoC
5Nrdquo unit
and putrescine a ldquoC4Nrdquo unit which are major components
for the biosynthesis of important alkaloids as will be discussed later (Scheme 16) Additionally ornithine is a precursor of arginine another important amino acid
ornithine‐derived alkaloids (incorporating the c4n
unit) Putrescine is derived from the decarboxylation of ornithine and is a precursor of linear polyamines like spermshyine After enzymatic methylation of one amine of putrescine
C10 C15
C15
C10
C10
C30
CO2H
Chrysanthemic acid
OH
Menthol
O
HO
H
HOGlc
MeO2CLoganin (iridoid)
H
H
H
HH
OO
OParthenolide
O
O
OHOHO
OAcHO
OnC3H7
O
O
O
nC7H15
Thapsigargin
O
O
OO
H
H
O
Cleavage
H
H
Artemisinin
C20 C40
O
O
OO
OO
OHOHH
Bilobalide(highly rearrangedand missing 5 C)
CleavageH
Highoxidativecleavages
HO OAcH
AcO O OH
OOOBz
O
Ph
BzNH
OHFrom PHE Paclitaxel
C25O
H
HO
OHCH
OH Ophiobolin A
H
HO
H
H
HHCholesterol
(missing 3 C WM)
H
H H H
H
H
Hopene
β-Carotene
C30 - 3
C15
C20 - 5WM
WM
FIGURE 13 Chemical diversity in the terpene series (Wm Wagnerndashmeerwein shifts lost carbons bold bonds are remnant of primary building blocks)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 7
in the presence of S‐adenosylmethionine transamination of the other affords γ‐(N‐methylamino)aldehyde [20] The resulting cyclic iminium is a key intermediate in the formation of many medicinally important alkaloids such as
the plant‐derived compounds cocaine atropine or the calysshytegines [21 22] Indeed this iminium is a mannich acceptor which can react with various nucleophiles the first of those being the carbanion of acetyl‐CoA Thus after a stepwise
CO2H
RCinnamic acid (R=H)p-Coumaric acid (R=OH)
RO
PHETYR
OH
[H] [O]
lignans
OH
O
O
O
O
OMeMeO
OMe
C mdash C andor C mdash Oradical couplings
After E Zisomerization
for example Podophyllotoxin
O ORO
Coumarins(eg scopoletin)
[O]
Prenylationcyclizationcleavage[O]
O OO
Furocoumarins(eg psoralen)
RO
nAcetyl-CoA
thencyclization
n = 2 styrylpyrones
O O
OMe
n = 3 avonoids(from chalcone synthase)
C6C3
H
H H
From AcCoA
O
n = 3 stilbenoids(from stilbene synthase)
HO
OH
OHFrom AcCoA From AcCoA(ndashCO2)
Resveratrol
HO
OH
OH
OH
OH
H
Catechin
MeO Yangonin
From AcCoA
SCHEME 15 The phenylpropanoid biosynthetic pathways
LYS
CO2
Cadaverine
O NH
H2O
NH
Piperideineiminium
Pelletierine
AcAcCoA
CO2
NH
O
N
O
Pseudopelletierine
NH
Ph
OH
Ph
O
Lobeline
NH
δ+
δndash
N
H
H
N
H
OH
Lupinine
N
H N
H
(+)-Sparteine
N
NH
O (ndash)-Cytisine
NH
CO2H
Pipecolic acid
N
OHHO
OH
HO
H
Castanos permine
ORN
CO2
NH2 NH2 NH2 NH2
NH2
NH2
NH2
PutrescineO
NHMe
N-Methylpyrrolinium
2 timesAcCoA
NMe
ARG
n = 1 spermidinen = 2 homospermidine
NMe
O
HygrineCO2
CO2
MeN
O
[O]
TropinoneCO2
HNHO
OHOH
OH
Calystegine B2
MeN
OAtropine
O
Ph
HO
NMe
NMe
NMe
O
Cuscohygrine
HN
HN
NH2
( )n
O
n = 2
N
HHO
Retronecine
HH
H
H
H
HO
H H
H
SCHEME 16 Lysine‐ and ornithine‐derived alkaloid biosynthetic pathways (mind the structural similarities)
8 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
elongation by two AcCoA units either decarboxylation can occur leading to the acetonylpyrrolidine hygrine or a secshyond mannich reaction by the intramolecular attack of the acetoacetate anion onto an oxidation‐derived pyrrolinium leading to the tropane skeleton (tropinone) The acetoacetyl‐CoA intermediate can also react intermolecularly with another pyrrolinium cation leading to cuscohygrine after decarboxylation Finally the pyrrolizidine alkaloids [23] are derived from homospermidine which when submitted to terminal oxidative deamination leads to the bicyclic skelshyeton of retronecine and further Senecio alkaloids We can mention herein that ornithine is a biosynthetic precursor of arginine bearing a guanidine function which is an intermediate toward the toxic compounds tetrodotoxin and saxitoxin (not shown)
lysine‐derived alkaloids (incorporating the c5n
unit) From lysine to piperidine alkaloids the biosynthetic steps parallel the one previously described from ornithine Indeed the oxidative deamination of cadaverine affords a δ‐amino aldehyde which cyclizes through imine formation into piperideine Protonation results in a mannich acceptor which is able to react with various nucleophiles such as β‐ketothioester anions The first product of these reactions is pelletierine which can further react through an intramolecular mannich
reaction leading to pseudopelletierine Quinolizidines [24] can also be formed first from the mannich reaction of the piperideine acceptor with the corresponding enamine nucleoshyphile and then after additional transformation steps leading for example to lupinine sparteine or cytisine
Indolizidine alkaloids [15] such as castanospermine and swainsonine are formed from pipecolic acid an amino acid derived from lysine which can be elongated by malonyl‐CoA followed by ring closure When protonated these alkaloids are oxonium mimics strongly inhibiting glycosidases
Tyrosine‐ and Phenylalanine‐Derived Alkaloids Tyrosine and phenylalanine amino acids are bearing the phenylshyethylamine moiety of many medicinally relevant alkaloids Further hydroxylations on the aromatic carbocycle or on the aliphatic part can be observed methylations can occur on phenolic oxygens and on the amine leading to catecholamines (adrenaline noradrenaline dopamine) Arylethylamines are also usual to react with endogenous aldehydes through PictetndashSpengler reactions [25] leading to important biosynthetic intermediates (Scheme 17) like
bull Reticuline from the reaction with 4‐hydroxyphenylacetshyaldehyde toward benzyltetrahydroisoquinoline alkaloids
NH2
Phenylethylamines
RONH TetrahydroisoquinolinesRO
RʹCHO
Rʹ
NH
(CH2)n
MeO
HO
Benzyltetrahydroisoquinoline(n = 1) for example reticuline
orPhenethyltetrahydroisoquinoline
(n = 2) eg autumnaline
IntermolecularCmdashO phenol
couplings
Curarealkaloids
IntramolecularC mdash C phenol
couplings
ONMe
HHO
H
HO
Morphine
NMe
MeO
HO
MeOOH
Isoboldine
O
O
OMe
NO2
CO2H
Aristolochic acid
OMe
O
NHAcMeO
MeOMeO
Colchicine
N
O
O
OMe
OMeBerberine
IntramolecularCmdash C coupling
and furtheroxidative events
Rʹ derivedfrom PHE
Pictet-Spenglerreaction
TYR
HO
HO CHO
HNHO
HO
OH
OMeO
OH
NMe
[O]
GalantamineNorbelladine
Rʹ derived fromsecologanin
NAc
HO
HO
O
CO2MeH
H
H
OHIpecosideaglycone
Ipecac alkaloids
1ClostCmdash C cleavage
and ring extension
H
ROH
1C lost
Cleavage
SCHEME 17 Tyrosine‐derived alkaloid biosynthetic pathways (double head arrows figure bond cleavages during biosynthetic processes)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 9
morphine berberine tubocurarine isoboldine or the highly modified aristolochic acid [26 27]
bull Automnaline from the reaction with 3‐(4‐hydroxyphenyl)propanal toward phenylethyltetrahydroisoquinoline alkaloids colchicine cephalotaxine or schelhammerishycine [28 29]
bull Ipecoside from the reaction of dopamine with secoloshyganin toward terpene tetrahydroisoquinoline alkaloids ipecoside or emetine
Lastly norbelladine (top of Scheme 17) is issued from the reductive amination of 34‐dihydroxybenzaldehyde (derived from phenylalanine) with tyramine (derived from tyrosine) and constitutes a biosynthetic node leading to Amaryllidaceae alkaloids such as galantamine crinine or lycorine depending on the topology of phenolic couplings In all these biosynthetic routes radical phenolic couplings are key reactions for CC and CO bond formations and rearrangements [30 31]
Tryptophan‐Derived Indole and Indole Monoterpene Alkaloids As for alkaloids derived from tyrosine and phenylalanine those derived from tryptophan are formed after decarboxylation of the amino acid (into tryptamine) and possible hydroxylation of the aromatic carbocycle (eg serotonin) and N‐methylation (eg psilocin) As previously tryptamine can also react through PictetndashSpengler reactions to form tetrahydro‐β‐carbolines which can be aromatized for example into harmine (Scheme 18) [16]
When the aldehyde partner of the PictetndashSpengler reacshytion with tryptamine is the terpene secologanin strictosidine is formed as an entry toward the vast monoterpene indole alkaloids [32 33] Hydrolysis of the glucosidic part releases the strictosidine aglycone bearing an aldehyde while iminshyium formation and further cyclization and reduction can lead to ajmalicine (from oxocyclization) or yohimbine (from carshybocyclization) These alkaloids are referred to as from the Corynanthe type with the monoterpene carbon skeleton unmodified Although it misses one carbon and has a very
RO
Carbonylcompounds
TRPNH
NH2
Indolethylamines(eg serotonine)
RONH
NH
Rʹ NH
N
MeOPictet-Spenglerreaction for example Harmineβ-Carbolines
NH
NHH
O
MeO2C
MeO2C
OH
H
H
Rʹ derived from secologanin
Strictosidine aglycone
NH
N
OH
H
H
Ajmalicine
N
N
O
O
H
H
H
Strychnine (C lost)Corynanthe type
Aspidosperma type Iboga type
N
N
OH
OMe
Quinine (C lost)
N
NH CO2Me
Catharanthine
NH
N
CO2MeH
OAc
OH
Vindoline
Complextransformations
NH
NMe
HN
MeN
H
H
Chimonanthine
Cyclizationdimerization
Prenylationcyclization
NH
NMeHO2C
H
Lysergic acid
H
H
Ergot alkaloidsPyrroloindoles Indole monoterpene
1C lost
1C lostFrom AcCoA
SCHEME 18 Tryptophan‐derived alkaloid biosynthetic pathways (gray parts monoterpenic units)
10 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
different structure strychnine is related to the Corynanthe alkaloids incorporating two carbons from acetyl‐CoA Highly modified monoterpene skeletons are derived from the Corynanthe core through CC bond breaking and reorshyganization leading to Iboga‐type (eg catharanthine) and Aspidosperma‐type (eg vindoline) alkaloids The antishycancer drug vinblastine is a heterodimer resulting from the nucleophilic attack of vindoline on a mannich acceptor resulting from catharanthine found in madagascar perishywinkle (Catharanthus roseus) The heteroaromatic comshypounds ellipticine camptothecin and quinine are also derived from a Corynanthe‐type precursor although in this case the biosynthetic relationship may not be obvious due to deep modifications of the skeleton
Finally two important classes of compounds have to be mentioned since they have inspired many synthetic chemists The pyrroloindole alkaloids result from the cyclization of tryptamine as found in physostigmine (formed by a cationic mechanism after methylation in position 3 of the indole not shown) or in chimonanthine (presumably formed by a radical coupling mechanism Scheme 18) The ergot alkaloids are derived from the 33‐dimethylallylation on position 4 of the indole in trypshytophan whose further cyclization and oxidation processes afford the natural products (eg lysergic acid Scheme 18 and ergotamine) which have had important medical applications [34]
NRPS Metabolites and Peptides NRPS enzymes assemble amino acids including nonproteinogenic ones into oligopeptides The enzymes contain several modules and especially an adenylation domain (A) which specifically selects and activates the amino acid to be transferred as a thioester on the nearby peptidyl carrier protein (PCP) [2] A condensation module (C) then catalyzes the formation of the peptide bonds between the newly introduced amino acyl‐PCP (bearing a free amine) and the elongated peptidyl‐PCP thioester At the end of the elongation a cyclization can occur into cyclopeptides but the peptide can also be
transferred to auxiliary enzymes like methyltransferases glycosyltransferases or oxidases (vancomycins are typical products of such functionalizations) [35 36] The formation of heterocycles is also frequently encountered in this metabolism as in penicillins that are derived from the tripeptide α‐aminoadipoyl‐cysteinyl‐valine or telomestatin (Fig 14) [2]
Other Alkaloid Origins There are many other nitrogen sources involved in alkaloid biosyntheses for example nicotinic acid (originated from aspartic acid and intershymediate in nicotine and anabasine biosyntheses) and anthranilic acid (originated from tryptophan and intermedishyate toward acridines or aurachins) The amination reaction (eg through transamination of carbonyl compounds) is also a way to introduce nitrogens in natural products for example from fatty acids steroids (toward Solanum alkaloids or cyclopamines) or other terpenoids (aconitine and atisine have diterpene skeletons while Daphniphyllum alkaloids are triterpene derivatives) Finally nucleic acids can also be precursors of alkaloids like the well‐known caffeine
12 FROM BIOSYNTHESIS TO TOTAL SYNTHESIS STRATEGIES TOWARD THE NATURAL PRODUCT CHEMICAL SPACE
121 The Chemical Space of Natural Products
Natural products occupy an important place in human com munities as demonstrated by their vast use from ancient times to nowadays like dyes fibers oils pershyfumes agrochemicals or drugs Broadly both primary and secondary metabolites could be classified as natural products while the latter as discussed previously are usushyally regarded as the ldquonatural productsrdquo owing to their comshyplexity and diversity arising from a variety of biosynthetic pathways The structural chemical diversity found among
N
S
CO2HO
HNPhH2C
O
Penicillin G
OH
HN
HN
O
O
ONH2
O
OHO
NHMe
OCl
O
NH
NH
NH
O
ClHO
O
HN
H
H
HOOH
OHHO2C
Vancomycin aglycone
ON
NO
O
NN
O
O
N
N O
Me
S
N N
OMeH
Telomestatin
L-AAA-L-CYS-D-VAL
Enzymes
FIGURE 14 Structural diversity of nonribosomal peptide compounds (AAA α‐aminoadipic acid)
FROm BIOSyNTHESIS TO TOTAL SyNTHESIS STRATEGIES TOWARD THE NATuRAL PRODuCT CHEmICAL SPACE 11
all living organisms defining the chemical space of natural products [37] is the consequence of their evolution occurshyring as an adaptation of organisms to their environment It is commonly believed that secondary metabolites are proshyduced as messengers by living organisms or as weapons against enemies and thus they should have certain biological activities in a medicinal point of view [38] Indeed natural products are regarded as one of the main sources of medicines (Fig 15) From the traditional medicinal extracts to every single bioactive molecule the methods of extraction purification identification and biological investigation of natural products have been well established Their complex structures and interesting properties have attracted synthetic chemists to accomplish their total syntheses and that of medicinally relevant anashylogs sometimes in the industrial context [39] Thus tarshygeting the chemical space of natural products has never been more relevant than today Although the discovery of natural products demands time and labor‐consuming manipulations it is worth to notice that the knowledge on this chemical space is still continually growing while biological advances allow for discovering and undershystanding potential targets However increasing the chemical space of human‐made compounds based on natural products should benefit from transdisciplinary colshylaborations such as the use of coupled biosynthetic and chemical synthetic methods to design original ldquounnatural natural productsrdquo [40]
122 The Biosynthetic Pathways as an Inspiration for Synthetic Challenges
1221 Precursor‐Directed Biosyntheses and Mutashysynthetic Strategies to Increase the Chemical Space of Natural Products As the genetics and biochemistry of natural product biosynthesis are better understood novel biosynthetic techniques have been developed to study and generate new diversity in natural product analogs (Scheme 19) Precursor‐directed biosynthesis (PDB) is considered as the earliest example of combining chemical and biological methods for the generation of complex natural product analogs [41 42] This approach compared with the biosynthetic pathway of wild‐type metabolites (Scheme 19a) involves the feeding of analogs of the natural biosynthetic building blocks to the living organisms (Scheme 19b) usually bacteria or fungi which incorporate the modified precursors into the biosynthesized compound mutasynthesis also termed as mutational biosynthesis (mBS) involves the inactivation of a key step of the bioshysynthesis in a mutant microorganism (Scheme 19c) which can then be fed by various modified or advanced building blocks (mutasynthons Scheme 19d) [43] These mutasshyynthons could not be incorporated by the wild type due to specificities of the enzymatic machinery Build up on PDB mBS eliminates the natural biosynthetic intermediate thus generating a less complex mixture of metabolites and making the purification or yield of target products better
O NH
OHO
O
OO
O
O
OH
HOO
O O
OH
HOO
NH ONH2
O
ONH
NH
O
OCl Cl
O
O
OH
HN
HN
HN
HN
OO
HO
HN
OH
OHOH
O
O
HO
HO
OHO
O
OHH2N
O
OH3C
H
O
H
CH3
CH3
H
O O
NO
H
O
HO
O
NO
O
NH
HN
OHO
HONH
ON
H
HO
O
H2N
O OH
ONH
OH
ON OHO
OH
HO OS OH
OOOH
OHO
O
ON
OO
O
HO
O
O OH
OO
Micafunginantifungal
Rapamycinimmunosuppressive
Galantamineanti-Alzheimer
Taxolanticancer
Artemisininantimalarial
Vancomycinantibiotic
FIGURE 15 Some famous natural products currently used as drugs
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 7
in the presence of S‐adenosylmethionine transamination of the other affords γ‐(N‐methylamino)aldehyde [20] The resulting cyclic iminium is a key intermediate in the formation of many medicinally important alkaloids such as
the plant‐derived compounds cocaine atropine or the calysshytegines [21 22] Indeed this iminium is a mannich acceptor which can react with various nucleophiles the first of those being the carbanion of acetyl‐CoA Thus after a stepwise
CO2H
RCinnamic acid (R=H)p-Coumaric acid (R=OH)
RO
PHETYR
OH
[H] [O]
lignans
OH
O
O
O
O
OMeMeO
OMe
C mdash C andor C mdash Oradical couplings
After E Zisomerization
for example Podophyllotoxin
O ORO
Coumarins(eg scopoletin)
[O]
Prenylationcyclizationcleavage[O]
O OO
Furocoumarins(eg psoralen)
RO
nAcetyl-CoA
thencyclization
n = 2 styrylpyrones
O O
OMe
n = 3 avonoids(from chalcone synthase)
C6C3
H
H H
From AcCoA
O
n = 3 stilbenoids(from stilbene synthase)
HO
OH
OHFrom AcCoA From AcCoA(ndashCO2)
Resveratrol
HO
OH
OH
OH
OH
H
Catechin
MeO Yangonin
From AcCoA
SCHEME 15 The phenylpropanoid biosynthetic pathways
LYS
CO2
Cadaverine
O NH
H2O
NH
Piperideineiminium
Pelletierine
AcAcCoA
CO2
NH
O
N
O
Pseudopelletierine
NH
Ph
OH
Ph
O
Lobeline
NH
δ+
δndash
N
H
H
N
H
OH
Lupinine
N
H N
H
(+)-Sparteine
N
NH
O (ndash)-Cytisine
NH
CO2H
Pipecolic acid
N
OHHO
OH
HO
H
Castanos permine
ORN
CO2
NH2 NH2 NH2 NH2
NH2
NH2
NH2
PutrescineO
NHMe
N-Methylpyrrolinium
2 timesAcCoA
NMe
ARG
n = 1 spermidinen = 2 homospermidine
NMe
O
HygrineCO2
CO2
MeN
O
[O]
TropinoneCO2
HNHO
OHOH
OH
Calystegine B2
MeN
OAtropine
O
Ph
HO
NMe
NMe
NMe
O
Cuscohygrine
HN
HN
NH2
( )n
O
n = 2
N
HHO
Retronecine
HH
H
H
H
HO
H H
H
SCHEME 16 Lysine‐ and ornithine‐derived alkaloid biosynthetic pathways (mind the structural similarities)
8 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
elongation by two AcCoA units either decarboxylation can occur leading to the acetonylpyrrolidine hygrine or a secshyond mannich reaction by the intramolecular attack of the acetoacetate anion onto an oxidation‐derived pyrrolinium leading to the tropane skeleton (tropinone) The acetoacetyl‐CoA intermediate can also react intermolecularly with another pyrrolinium cation leading to cuscohygrine after decarboxylation Finally the pyrrolizidine alkaloids [23] are derived from homospermidine which when submitted to terminal oxidative deamination leads to the bicyclic skelshyeton of retronecine and further Senecio alkaloids We can mention herein that ornithine is a biosynthetic precursor of arginine bearing a guanidine function which is an intermediate toward the toxic compounds tetrodotoxin and saxitoxin (not shown)
lysine‐derived alkaloids (incorporating the c5n
unit) From lysine to piperidine alkaloids the biosynthetic steps parallel the one previously described from ornithine Indeed the oxidative deamination of cadaverine affords a δ‐amino aldehyde which cyclizes through imine formation into piperideine Protonation results in a mannich acceptor which is able to react with various nucleophiles such as β‐ketothioester anions The first product of these reactions is pelletierine which can further react through an intramolecular mannich
reaction leading to pseudopelletierine Quinolizidines [24] can also be formed first from the mannich reaction of the piperideine acceptor with the corresponding enamine nucleoshyphile and then after additional transformation steps leading for example to lupinine sparteine or cytisine
Indolizidine alkaloids [15] such as castanospermine and swainsonine are formed from pipecolic acid an amino acid derived from lysine which can be elongated by malonyl‐CoA followed by ring closure When protonated these alkaloids are oxonium mimics strongly inhibiting glycosidases
Tyrosine‐ and Phenylalanine‐Derived Alkaloids Tyrosine and phenylalanine amino acids are bearing the phenylshyethylamine moiety of many medicinally relevant alkaloids Further hydroxylations on the aromatic carbocycle or on the aliphatic part can be observed methylations can occur on phenolic oxygens and on the amine leading to catecholamines (adrenaline noradrenaline dopamine) Arylethylamines are also usual to react with endogenous aldehydes through PictetndashSpengler reactions [25] leading to important biosynthetic intermediates (Scheme 17) like
bull Reticuline from the reaction with 4‐hydroxyphenylacetshyaldehyde toward benzyltetrahydroisoquinoline alkaloids
NH2
Phenylethylamines
RONH TetrahydroisoquinolinesRO
RʹCHO
Rʹ
NH
(CH2)n
MeO
HO
Benzyltetrahydroisoquinoline(n = 1) for example reticuline
orPhenethyltetrahydroisoquinoline
(n = 2) eg autumnaline
IntermolecularCmdashO phenol
couplings
Curarealkaloids
IntramolecularC mdash C phenol
couplings
ONMe
HHO
H
HO
Morphine
NMe
MeO
HO
MeOOH
Isoboldine
O
O
OMe
NO2
CO2H
Aristolochic acid
OMe
O
NHAcMeO
MeOMeO
Colchicine
N
O
O
OMe
OMeBerberine
IntramolecularCmdash C coupling
and furtheroxidative events
Rʹ derivedfrom PHE
Pictet-Spenglerreaction
TYR
HO
HO CHO
HNHO
HO
OH
OMeO
OH
NMe
[O]
GalantamineNorbelladine
Rʹ derived fromsecologanin
NAc
HO
HO
O
CO2MeH
H
H
OHIpecosideaglycone
Ipecac alkaloids
1ClostCmdash C cleavage
and ring extension
H
ROH
1C lost
Cleavage
SCHEME 17 Tyrosine‐derived alkaloid biosynthetic pathways (double head arrows figure bond cleavages during biosynthetic processes)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 9
morphine berberine tubocurarine isoboldine or the highly modified aristolochic acid [26 27]
bull Automnaline from the reaction with 3‐(4‐hydroxyphenyl)propanal toward phenylethyltetrahydroisoquinoline alkaloids colchicine cephalotaxine or schelhammerishycine [28 29]
bull Ipecoside from the reaction of dopamine with secoloshyganin toward terpene tetrahydroisoquinoline alkaloids ipecoside or emetine
Lastly norbelladine (top of Scheme 17) is issued from the reductive amination of 34‐dihydroxybenzaldehyde (derived from phenylalanine) with tyramine (derived from tyrosine) and constitutes a biosynthetic node leading to Amaryllidaceae alkaloids such as galantamine crinine or lycorine depending on the topology of phenolic couplings In all these biosynthetic routes radical phenolic couplings are key reactions for CC and CO bond formations and rearrangements [30 31]
Tryptophan‐Derived Indole and Indole Monoterpene Alkaloids As for alkaloids derived from tyrosine and phenylalanine those derived from tryptophan are formed after decarboxylation of the amino acid (into tryptamine) and possible hydroxylation of the aromatic carbocycle (eg serotonin) and N‐methylation (eg psilocin) As previously tryptamine can also react through PictetndashSpengler reactions to form tetrahydro‐β‐carbolines which can be aromatized for example into harmine (Scheme 18) [16]
When the aldehyde partner of the PictetndashSpengler reacshytion with tryptamine is the terpene secologanin strictosidine is formed as an entry toward the vast monoterpene indole alkaloids [32 33] Hydrolysis of the glucosidic part releases the strictosidine aglycone bearing an aldehyde while iminshyium formation and further cyclization and reduction can lead to ajmalicine (from oxocyclization) or yohimbine (from carshybocyclization) These alkaloids are referred to as from the Corynanthe type with the monoterpene carbon skeleton unmodified Although it misses one carbon and has a very
RO
Carbonylcompounds
TRPNH
NH2
Indolethylamines(eg serotonine)
RONH
NH
Rʹ NH
N
MeOPictet-Spenglerreaction for example Harmineβ-Carbolines
NH
NHH
O
MeO2C
MeO2C
OH
H
H
Rʹ derived from secologanin
Strictosidine aglycone
NH
N
OH
H
H
Ajmalicine
N
N
O
O
H
H
H
Strychnine (C lost)Corynanthe type
Aspidosperma type Iboga type
N
N
OH
OMe
Quinine (C lost)
N
NH CO2Me
Catharanthine
NH
N
CO2MeH
OAc
OH
Vindoline
Complextransformations
NH
NMe
HN
MeN
H
H
Chimonanthine
Cyclizationdimerization
Prenylationcyclization
NH
NMeHO2C
H
Lysergic acid
H
H
Ergot alkaloidsPyrroloindoles Indole monoterpene
1C lost
1C lostFrom AcCoA
SCHEME 18 Tryptophan‐derived alkaloid biosynthetic pathways (gray parts monoterpenic units)
10 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
different structure strychnine is related to the Corynanthe alkaloids incorporating two carbons from acetyl‐CoA Highly modified monoterpene skeletons are derived from the Corynanthe core through CC bond breaking and reorshyganization leading to Iboga‐type (eg catharanthine) and Aspidosperma‐type (eg vindoline) alkaloids The antishycancer drug vinblastine is a heterodimer resulting from the nucleophilic attack of vindoline on a mannich acceptor resulting from catharanthine found in madagascar perishywinkle (Catharanthus roseus) The heteroaromatic comshypounds ellipticine camptothecin and quinine are also derived from a Corynanthe‐type precursor although in this case the biosynthetic relationship may not be obvious due to deep modifications of the skeleton
Finally two important classes of compounds have to be mentioned since they have inspired many synthetic chemists The pyrroloindole alkaloids result from the cyclization of tryptamine as found in physostigmine (formed by a cationic mechanism after methylation in position 3 of the indole not shown) or in chimonanthine (presumably formed by a radical coupling mechanism Scheme 18) The ergot alkaloids are derived from the 33‐dimethylallylation on position 4 of the indole in trypshytophan whose further cyclization and oxidation processes afford the natural products (eg lysergic acid Scheme 18 and ergotamine) which have had important medical applications [34]
NRPS Metabolites and Peptides NRPS enzymes assemble amino acids including nonproteinogenic ones into oligopeptides The enzymes contain several modules and especially an adenylation domain (A) which specifically selects and activates the amino acid to be transferred as a thioester on the nearby peptidyl carrier protein (PCP) [2] A condensation module (C) then catalyzes the formation of the peptide bonds between the newly introduced amino acyl‐PCP (bearing a free amine) and the elongated peptidyl‐PCP thioester At the end of the elongation a cyclization can occur into cyclopeptides but the peptide can also be
transferred to auxiliary enzymes like methyltransferases glycosyltransferases or oxidases (vancomycins are typical products of such functionalizations) [35 36] The formation of heterocycles is also frequently encountered in this metabolism as in penicillins that are derived from the tripeptide α‐aminoadipoyl‐cysteinyl‐valine or telomestatin (Fig 14) [2]
Other Alkaloid Origins There are many other nitrogen sources involved in alkaloid biosyntheses for example nicotinic acid (originated from aspartic acid and intershymediate in nicotine and anabasine biosyntheses) and anthranilic acid (originated from tryptophan and intermedishyate toward acridines or aurachins) The amination reaction (eg through transamination of carbonyl compounds) is also a way to introduce nitrogens in natural products for example from fatty acids steroids (toward Solanum alkaloids or cyclopamines) or other terpenoids (aconitine and atisine have diterpene skeletons while Daphniphyllum alkaloids are triterpene derivatives) Finally nucleic acids can also be precursors of alkaloids like the well‐known caffeine
12 FROM BIOSYNTHESIS TO TOTAL SYNTHESIS STRATEGIES TOWARD THE NATURAL PRODUCT CHEMICAL SPACE
121 The Chemical Space of Natural Products
Natural products occupy an important place in human com munities as demonstrated by their vast use from ancient times to nowadays like dyes fibers oils pershyfumes agrochemicals or drugs Broadly both primary and secondary metabolites could be classified as natural products while the latter as discussed previously are usushyally regarded as the ldquonatural productsrdquo owing to their comshyplexity and diversity arising from a variety of biosynthetic pathways The structural chemical diversity found among
N
S
CO2HO
HNPhH2C
O
Penicillin G
OH
HN
HN
O
O
ONH2
O
OHO
NHMe
OCl
O
NH
NH
NH
O
ClHO
O
HN
H
H
HOOH
OHHO2C
Vancomycin aglycone
ON
NO
O
NN
O
O
N
N O
Me
S
N N
OMeH
Telomestatin
L-AAA-L-CYS-D-VAL
Enzymes
FIGURE 14 Structural diversity of nonribosomal peptide compounds (AAA α‐aminoadipic acid)
FROm BIOSyNTHESIS TO TOTAL SyNTHESIS STRATEGIES TOWARD THE NATuRAL PRODuCT CHEmICAL SPACE 11
all living organisms defining the chemical space of natural products [37] is the consequence of their evolution occurshyring as an adaptation of organisms to their environment It is commonly believed that secondary metabolites are proshyduced as messengers by living organisms or as weapons against enemies and thus they should have certain biological activities in a medicinal point of view [38] Indeed natural products are regarded as one of the main sources of medicines (Fig 15) From the traditional medicinal extracts to every single bioactive molecule the methods of extraction purification identification and biological investigation of natural products have been well established Their complex structures and interesting properties have attracted synthetic chemists to accomplish their total syntheses and that of medicinally relevant anashylogs sometimes in the industrial context [39] Thus tarshygeting the chemical space of natural products has never been more relevant than today Although the discovery of natural products demands time and labor‐consuming manipulations it is worth to notice that the knowledge on this chemical space is still continually growing while biological advances allow for discovering and undershystanding potential targets However increasing the chemical space of human‐made compounds based on natural products should benefit from transdisciplinary colshylaborations such as the use of coupled biosynthetic and chemical synthetic methods to design original ldquounnatural natural productsrdquo [40]
122 The Biosynthetic Pathways as an Inspiration for Synthetic Challenges
1221 Precursor‐Directed Biosyntheses and Mutashysynthetic Strategies to Increase the Chemical Space of Natural Products As the genetics and biochemistry of natural product biosynthesis are better understood novel biosynthetic techniques have been developed to study and generate new diversity in natural product analogs (Scheme 19) Precursor‐directed biosynthesis (PDB) is considered as the earliest example of combining chemical and biological methods for the generation of complex natural product analogs [41 42] This approach compared with the biosynthetic pathway of wild‐type metabolites (Scheme 19a) involves the feeding of analogs of the natural biosynthetic building blocks to the living organisms (Scheme 19b) usually bacteria or fungi which incorporate the modified precursors into the biosynthesized compound mutasynthesis also termed as mutational biosynthesis (mBS) involves the inactivation of a key step of the bioshysynthesis in a mutant microorganism (Scheme 19c) which can then be fed by various modified or advanced building blocks (mutasynthons Scheme 19d) [43] These mutasshyynthons could not be incorporated by the wild type due to specificities of the enzymatic machinery Build up on PDB mBS eliminates the natural biosynthetic intermediate thus generating a less complex mixture of metabolites and making the purification or yield of target products better
O NH
OHO
O
OO
O
O
OH
HOO
O O
OH
HOO
NH ONH2
O
ONH
NH
O
OCl Cl
O
O
OH
HN
HN
HN
HN
OO
HO
HN
OH
OHOH
O
O
HO
HO
OHO
O
OHH2N
O
OH3C
H
O
H
CH3
CH3
H
O O
NO
H
O
HO
O
NO
O
NH
HN
OHO
HONH
ON
H
HO
O
H2N
O OH
ONH
OH
ON OHO
OH
HO OS OH
OOOH
OHO
O
ON
OO
O
HO
O
O OH
OO
Micafunginantifungal
Rapamycinimmunosuppressive
Galantamineanti-Alzheimer
Taxolanticancer
Artemisininantimalarial
Vancomycinantibiotic
FIGURE 15 Some famous natural products currently used as drugs
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product
8 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
elongation by two AcCoA units either decarboxylation can occur leading to the acetonylpyrrolidine hygrine or a secshyond mannich reaction by the intramolecular attack of the acetoacetate anion onto an oxidation‐derived pyrrolinium leading to the tropane skeleton (tropinone) The acetoacetyl‐CoA intermediate can also react intermolecularly with another pyrrolinium cation leading to cuscohygrine after decarboxylation Finally the pyrrolizidine alkaloids [23] are derived from homospermidine which when submitted to terminal oxidative deamination leads to the bicyclic skelshyeton of retronecine and further Senecio alkaloids We can mention herein that ornithine is a biosynthetic precursor of arginine bearing a guanidine function which is an intermediate toward the toxic compounds tetrodotoxin and saxitoxin (not shown)
lysine‐derived alkaloids (incorporating the c5n
unit) From lysine to piperidine alkaloids the biosynthetic steps parallel the one previously described from ornithine Indeed the oxidative deamination of cadaverine affords a δ‐amino aldehyde which cyclizes through imine formation into piperideine Protonation results in a mannich acceptor which is able to react with various nucleophiles such as β‐ketothioester anions The first product of these reactions is pelletierine which can further react through an intramolecular mannich
reaction leading to pseudopelletierine Quinolizidines [24] can also be formed first from the mannich reaction of the piperideine acceptor with the corresponding enamine nucleoshyphile and then after additional transformation steps leading for example to lupinine sparteine or cytisine
Indolizidine alkaloids [15] such as castanospermine and swainsonine are formed from pipecolic acid an amino acid derived from lysine which can be elongated by malonyl‐CoA followed by ring closure When protonated these alkaloids are oxonium mimics strongly inhibiting glycosidases
Tyrosine‐ and Phenylalanine‐Derived Alkaloids Tyrosine and phenylalanine amino acids are bearing the phenylshyethylamine moiety of many medicinally relevant alkaloids Further hydroxylations on the aromatic carbocycle or on the aliphatic part can be observed methylations can occur on phenolic oxygens and on the amine leading to catecholamines (adrenaline noradrenaline dopamine) Arylethylamines are also usual to react with endogenous aldehydes through PictetndashSpengler reactions [25] leading to important biosynthetic intermediates (Scheme 17) like
bull Reticuline from the reaction with 4‐hydroxyphenylacetshyaldehyde toward benzyltetrahydroisoquinoline alkaloids
NH2
Phenylethylamines
RONH TetrahydroisoquinolinesRO
RʹCHO
Rʹ
NH
(CH2)n
MeO
HO
Benzyltetrahydroisoquinoline(n = 1) for example reticuline
orPhenethyltetrahydroisoquinoline
(n = 2) eg autumnaline
IntermolecularCmdashO phenol
couplings
Curarealkaloids
IntramolecularC mdash C phenol
couplings
ONMe
HHO
H
HO
Morphine
NMe
MeO
HO
MeOOH
Isoboldine
O
O
OMe
NO2
CO2H
Aristolochic acid
OMe
O
NHAcMeO
MeOMeO
Colchicine
N
O
O
OMe
OMeBerberine
IntramolecularCmdash C coupling
and furtheroxidative events
Rʹ derivedfrom PHE
Pictet-Spenglerreaction
TYR
HO
HO CHO
HNHO
HO
OH
OMeO
OH
NMe
[O]
GalantamineNorbelladine
Rʹ derived fromsecologanin
NAc
HO
HO
O
CO2MeH
H
H
OHIpecosideaglycone
Ipecac alkaloids
1ClostCmdash C cleavage
and ring extension
H
ROH
1C lost
Cleavage
SCHEME 17 Tyrosine‐derived alkaloid biosynthetic pathways (double head arrows figure bond cleavages during biosynthetic processes)
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 9
morphine berberine tubocurarine isoboldine or the highly modified aristolochic acid [26 27]
bull Automnaline from the reaction with 3‐(4‐hydroxyphenyl)propanal toward phenylethyltetrahydroisoquinoline alkaloids colchicine cephalotaxine or schelhammerishycine [28 29]
bull Ipecoside from the reaction of dopamine with secoloshyganin toward terpene tetrahydroisoquinoline alkaloids ipecoside or emetine
Lastly norbelladine (top of Scheme 17) is issued from the reductive amination of 34‐dihydroxybenzaldehyde (derived from phenylalanine) with tyramine (derived from tyrosine) and constitutes a biosynthetic node leading to Amaryllidaceae alkaloids such as galantamine crinine or lycorine depending on the topology of phenolic couplings In all these biosynthetic routes radical phenolic couplings are key reactions for CC and CO bond formations and rearrangements [30 31]
Tryptophan‐Derived Indole and Indole Monoterpene Alkaloids As for alkaloids derived from tyrosine and phenylalanine those derived from tryptophan are formed after decarboxylation of the amino acid (into tryptamine) and possible hydroxylation of the aromatic carbocycle (eg serotonin) and N‐methylation (eg psilocin) As previously tryptamine can also react through PictetndashSpengler reactions to form tetrahydro‐β‐carbolines which can be aromatized for example into harmine (Scheme 18) [16]
When the aldehyde partner of the PictetndashSpengler reacshytion with tryptamine is the terpene secologanin strictosidine is formed as an entry toward the vast monoterpene indole alkaloids [32 33] Hydrolysis of the glucosidic part releases the strictosidine aglycone bearing an aldehyde while iminshyium formation and further cyclization and reduction can lead to ajmalicine (from oxocyclization) or yohimbine (from carshybocyclization) These alkaloids are referred to as from the Corynanthe type with the monoterpene carbon skeleton unmodified Although it misses one carbon and has a very
RO
Carbonylcompounds
TRPNH
NH2
Indolethylamines(eg serotonine)
RONH
NH
Rʹ NH
N
MeOPictet-Spenglerreaction for example Harmineβ-Carbolines
NH
NHH
O
MeO2C
MeO2C
OH
H
H
Rʹ derived from secologanin
Strictosidine aglycone
NH
N
OH
H
H
Ajmalicine
N
N
O
O
H
H
H
Strychnine (C lost)Corynanthe type
Aspidosperma type Iboga type
N
N
OH
OMe
Quinine (C lost)
N
NH CO2Me
Catharanthine
NH
N
CO2MeH
OAc
OH
Vindoline
Complextransformations
NH
NMe
HN
MeN
H
H
Chimonanthine
Cyclizationdimerization
Prenylationcyclization
NH
NMeHO2C
H
Lysergic acid
H
H
Ergot alkaloidsPyrroloindoles Indole monoterpene
1C lost
1C lostFrom AcCoA
SCHEME 18 Tryptophan‐derived alkaloid biosynthetic pathways (gray parts monoterpenic units)
10 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
different structure strychnine is related to the Corynanthe alkaloids incorporating two carbons from acetyl‐CoA Highly modified monoterpene skeletons are derived from the Corynanthe core through CC bond breaking and reorshyganization leading to Iboga‐type (eg catharanthine) and Aspidosperma‐type (eg vindoline) alkaloids The antishycancer drug vinblastine is a heterodimer resulting from the nucleophilic attack of vindoline on a mannich acceptor resulting from catharanthine found in madagascar perishywinkle (Catharanthus roseus) The heteroaromatic comshypounds ellipticine camptothecin and quinine are also derived from a Corynanthe‐type precursor although in this case the biosynthetic relationship may not be obvious due to deep modifications of the skeleton
Finally two important classes of compounds have to be mentioned since they have inspired many synthetic chemists The pyrroloindole alkaloids result from the cyclization of tryptamine as found in physostigmine (formed by a cationic mechanism after methylation in position 3 of the indole not shown) or in chimonanthine (presumably formed by a radical coupling mechanism Scheme 18) The ergot alkaloids are derived from the 33‐dimethylallylation on position 4 of the indole in trypshytophan whose further cyclization and oxidation processes afford the natural products (eg lysergic acid Scheme 18 and ergotamine) which have had important medical applications [34]
NRPS Metabolites and Peptides NRPS enzymes assemble amino acids including nonproteinogenic ones into oligopeptides The enzymes contain several modules and especially an adenylation domain (A) which specifically selects and activates the amino acid to be transferred as a thioester on the nearby peptidyl carrier protein (PCP) [2] A condensation module (C) then catalyzes the formation of the peptide bonds between the newly introduced amino acyl‐PCP (bearing a free amine) and the elongated peptidyl‐PCP thioester At the end of the elongation a cyclization can occur into cyclopeptides but the peptide can also be
transferred to auxiliary enzymes like methyltransferases glycosyltransferases or oxidases (vancomycins are typical products of such functionalizations) [35 36] The formation of heterocycles is also frequently encountered in this metabolism as in penicillins that are derived from the tripeptide α‐aminoadipoyl‐cysteinyl‐valine or telomestatin (Fig 14) [2]
Other Alkaloid Origins There are many other nitrogen sources involved in alkaloid biosyntheses for example nicotinic acid (originated from aspartic acid and intershymediate in nicotine and anabasine biosyntheses) and anthranilic acid (originated from tryptophan and intermedishyate toward acridines or aurachins) The amination reaction (eg through transamination of carbonyl compounds) is also a way to introduce nitrogens in natural products for example from fatty acids steroids (toward Solanum alkaloids or cyclopamines) or other terpenoids (aconitine and atisine have diterpene skeletons while Daphniphyllum alkaloids are triterpene derivatives) Finally nucleic acids can also be precursors of alkaloids like the well‐known caffeine
12 FROM BIOSYNTHESIS TO TOTAL SYNTHESIS STRATEGIES TOWARD THE NATURAL PRODUCT CHEMICAL SPACE
121 The Chemical Space of Natural Products
Natural products occupy an important place in human com munities as demonstrated by their vast use from ancient times to nowadays like dyes fibers oils pershyfumes agrochemicals or drugs Broadly both primary and secondary metabolites could be classified as natural products while the latter as discussed previously are usushyally regarded as the ldquonatural productsrdquo owing to their comshyplexity and diversity arising from a variety of biosynthetic pathways The structural chemical diversity found among
N
S
CO2HO
HNPhH2C
O
Penicillin G
OH
HN
HN
O
O
ONH2
O
OHO
NHMe
OCl
O
NH
NH
NH
O
ClHO
O
HN
H
H
HOOH
OHHO2C
Vancomycin aglycone
ON
NO
O
NN
O
O
N
N O
Me
S
N N
OMeH
Telomestatin
L-AAA-L-CYS-D-VAL
Enzymes
FIGURE 14 Structural diversity of nonribosomal peptide compounds (AAA α‐aminoadipic acid)
FROm BIOSyNTHESIS TO TOTAL SyNTHESIS STRATEGIES TOWARD THE NATuRAL PRODuCT CHEmICAL SPACE 11
all living organisms defining the chemical space of natural products [37] is the consequence of their evolution occurshyring as an adaptation of organisms to their environment It is commonly believed that secondary metabolites are proshyduced as messengers by living organisms or as weapons against enemies and thus they should have certain biological activities in a medicinal point of view [38] Indeed natural products are regarded as one of the main sources of medicines (Fig 15) From the traditional medicinal extracts to every single bioactive molecule the methods of extraction purification identification and biological investigation of natural products have been well established Their complex structures and interesting properties have attracted synthetic chemists to accomplish their total syntheses and that of medicinally relevant anashylogs sometimes in the industrial context [39] Thus tarshygeting the chemical space of natural products has never been more relevant than today Although the discovery of natural products demands time and labor‐consuming manipulations it is worth to notice that the knowledge on this chemical space is still continually growing while biological advances allow for discovering and undershystanding potential targets However increasing the chemical space of human‐made compounds based on natural products should benefit from transdisciplinary colshylaborations such as the use of coupled biosynthetic and chemical synthetic methods to design original ldquounnatural natural productsrdquo [40]
122 The Biosynthetic Pathways as an Inspiration for Synthetic Challenges
1221 Precursor‐Directed Biosyntheses and Mutashysynthetic Strategies to Increase the Chemical Space of Natural Products As the genetics and biochemistry of natural product biosynthesis are better understood novel biosynthetic techniques have been developed to study and generate new diversity in natural product analogs (Scheme 19) Precursor‐directed biosynthesis (PDB) is considered as the earliest example of combining chemical and biological methods for the generation of complex natural product analogs [41 42] This approach compared with the biosynthetic pathway of wild‐type metabolites (Scheme 19a) involves the feeding of analogs of the natural biosynthetic building blocks to the living organisms (Scheme 19b) usually bacteria or fungi which incorporate the modified precursors into the biosynthesized compound mutasynthesis also termed as mutational biosynthesis (mBS) involves the inactivation of a key step of the bioshysynthesis in a mutant microorganism (Scheme 19c) which can then be fed by various modified or advanced building blocks (mutasynthons Scheme 19d) [43] These mutasshyynthons could not be incorporated by the wild type due to specificities of the enzymatic machinery Build up on PDB mBS eliminates the natural biosynthetic intermediate thus generating a less complex mixture of metabolites and making the purification or yield of target products better
O NH
OHO
O
OO
O
O
OH
HOO
O O
OH
HOO
NH ONH2
O
ONH
NH
O
OCl Cl
O
O
OH
HN
HN
HN
HN
OO
HO
HN
OH
OHOH
O
O
HO
HO
OHO
O
OHH2N
O
OH3C
H
O
H
CH3
CH3
H
O O
NO
H
O
HO
O
NO
O
NH
HN
OHO
HONH
ON
H
HO
O
H2N
O OH
ONH
OH
ON OHO
OH
HO OS OH
OOOH
OHO
O
ON
OO
O
HO
O
O OH
OO
Micafunginantifungal
Rapamycinimmunosuppressive
Galantamineanti-Alzheimer
Taxolanticancer
Artemisininantimalarial
Vancomycinantibiotic
FIGURE 15 Some famous natural products currently used as drugs
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product
FROm PRImARy TO SECONDARy mETABOLISm THE KEy BuILDING BLOCKS 9
morphine berberine tubocurarine isoboldine or the highly modified aristolochic acid [26 27]
bull Automnaline from the reaction with 3‐(4‐hydroxyphenyl)propanal toward phenylethyltetrahydroisoquinoline alkaloids colchicine cephalotaxine or schelhammerishycine [28 29]
bull Ipecoside from the reaction of dopamine with secoloshyganin toward terpene tetrahydroisoquinoline alkaloids ipecoside or emetine
Lastly norbelladine (top of Scheme 17) is issued from the reductive amination of 34‐dihydroxybenzaldehyde (derived from phenylalanine) with tyramine (derived from tyrosine) and constitutes a biosynthetic node leading to Amaryllidaceae alkaloids such as galantamine crinine or lycorine depending on the topology of phenolic couplings In all these biosynthetic routes radical phenolic couplings are key reactions for CC and CO bond formations and rearrangements [30 31]
Tryptophan‐Derived Indole and Indole Monoterpene Alkaloids As for alkaloids derived from tyrosine and phenylalanine those derived from tryptophan are formed after decarboxylation of the amino acid (into tryptamine) and possible hydroxylation of the aromatic carbocycle (eg serotonin) and N‐methylation (eg psilocin) As previously tryptamine can also react through PictetndashSpengler reactions to form tetrahydro‐β‐carbolines which can be aromatized for example into harmine (Scheme 18) [16]
When the aldehyde partner of the PictetndashSpengler reacshytion with tryptamine is the terpene secologanin strictosidine is formed as an entry toward the vast monoterpene indole alkaloids [32 33] Hydrolysis of the glucosidic part releases the strictosidine aglycone bearing an aldehyde while iminshyium formation and further cyclization and reduction can lead to ajmalicine (from oxocyclization) or yohimbine (from carshybocyclization) These alkaloids are referred to as from the Corynanthe type with the monoterpene carbon skeleton unmodified Although it misses one carbon and has a very
RO
Carbonylcompounds
TRPNH
NH2
Indolethylamines(eg serotonine)
RONH
NH
Rʹ NH
N
MeOPictet-Spenglerreaction for example Harmineβ-Carbolines
NH
NHH
O
MeO2C
MeO2C
OH
H
H
Rʹ derived from secologanin
Strictosidine aglycone
NH
N
OH
H
H
Ajmalicine
N
N
O
O
H
H
H
Strychnine (C lost)Corynanthe type
Aspidosperma type Iboga type
N
N
OH
OMe
Quinine (C lost)
N
NH CO2Me
Catharanthine
NH
N
CO2MeH
OAc
OH
Vindoline
Complextransformations
NH
NMe
HN
MeN
H
H
Chimonanthine
Cyclizationdimerization
Prenylationcyclization
NH
NMeHO2C
H
Lysergic acid
H
H
Ergot alkaloidsPyrroloindoles Indole monoterpene
1C lost
1C lostFrom AcCoA
SCHEME 18 Tryptophan‐derived alkaloid biosynthetic pathways (gray parts monoterpenic units)
10 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
different structure strychnine is related to the Corynanthe alkaloids incorporating two carbons from acetyl‐CoA Highly modified monoterpene skeletons are derived from the Corynanthe core through CC bond breaking and reorshyganization leading to Iboga‐type (eg catharanthine) and Aspidosperma‐type (eg vindoline) alkaloids The antishycancer drug vinblastine is a heterodimer resulting from the nucleophilic attack of vindoline on a mannich acceptor resulting from catharanthine found in madagascar perishywinkle (Catharanthus roseus) The heteroaromatic comshypounds ellipticine camptothecin and quinine are also derived from a Corynanthe‐type precursor although in this case the biosynthetic relationship may not be obvious due to deep modifications of the skeleton
Finally two important classes of compounds have to be mentioned since they have inspired many synthetic chemists The pyrroloindole alkaloids result from the cyclization of tryptamine as found in physostigmine (formed by a cationic mechanism after methylation in position 3 of the indole not shown) or in chimonanthine (presumably formed by a radical coupling mechanism Scheme 18) The ergot alkaloids are derived from the 33‐dimethylallylation on position 4 of the indole in trypshytophan whose further cyclization and oxidation processes afford the natural products (eg lysergic acid Scheme 18 and ergotamine) which have had important medical applications [34]
NRPS Metabolites and Peptides NRPS enzymes assemble amino acids including nonproteinogenic ones into oligopeptides The enzymes contain several modules and especially an adenylation domain (A) which specifically selects and activates the amino acid to be transferred as a thioester on the nearby peptidyl carrier protein (PCP) [2] A condensation module (C) then catalyzes the formation of the peptide bonds between the newly introduced amino acyl‐PCP (bearing a free amine) and the elongated peptidyl‐PCP thioester At the end of the elongation a cyclization can occur into cyclopeptides but the peptide can also be
transferred to auxiliary enzymes like methyltransferases glycosyltransferases or oxidases (vancomycins are typical products of such functionalizations) [35 36] The formation of heterocycles is also frequently encountered in this metabolism as in penicillins that are derived from the tripeptide α‐aminoadipoyl‐cysteinyl‐valine or telomestatin (Fig 14) [2]
Other Alkaloid Origins There are many other nitrogen sources involved in alkaloid biosyntheses for example nicotinic acid (originated from aspartic acid and intershymediate in nicotine and anabasine biosyntheses) and anthranilic acid (originated from tryptophan and intermedishyate toward acridines or aurachins) The amination reaction (eg through transamination of carbonyl compounds) is also a way to introduce nitrogens in natural products for example from fatty acids steroids (toward Solanum alkaloids or cyclopamines) or other terpenoids (aconitine and atisine have diterpene skeletons while Daphniphyllum alkaloids are triterpene derivatives) Finally nucleic acids can also be precursors of alkaloids like the well‐known caffeine
12 FROM BIOSYNTHESIS TO TOTAL SYNTHESIS STRATEGIES TOWARD THE NATURAL PRODUCT CHEMICAL SPACE
121 The Chemical Space of Natural Products
Natural products occupy an important place in human com munities as demonstrated by their vast use from ancient times to nowadays like dyes fibers oils pershyfumes agrochemicals or drugs Broadly both primary and secondary metabolites could be classified as natural products while the latter as discussed previously are usushyally regarded as the ldquonatural productsrdquo owing to their comshyplexity and diversity arising from a variety of biosynthetic pathways The structural chemical diversity found among
N
S
CO2HO
HNPhH2C
O
Penicillin G
OH
HN
HN
O
O
ONH2
O
OHO
NHMe
OCl
O
NH
NH
NH
O
ClHO
O
HN
H
H
HOOH
OHHO2C
Vancomycin aglycone
ON
NO
O
NN
O
O
N
N O
Me
S
N N
OMeH
Telomestatin
L-AAA-L-CYS-D-VAL
Enzymes
FIGURE 14 Structural diversity of nonribosomal peptide compounds (AAA α‐aminoadipic acid)
FROm BIOSyNTHESIS TO TOTAL SyNTHESIS STRATEGIES TOWARD THE NATuRAL PRODuCT CHEmICAL SPACE 11
all living organisms defining the chemical space of natural products [37] is the consequence of their evolution occurshyring as an adaptation of organisms to their environment It is commonly believed that secondary metabolites are proshyduced as messengers by living organisms or as weapons against enemies and thus they should have certain biological activities in a medicinal point of view [38] Indeed natural products are regarded as one of the main sources of medicines (Fig 15) From the traditional medicinal extracts to every single bioactive molecule the methods of extraction purification identification and biological investigation of natural products have been well established Their complex structures and interesting properties have attracted synthetic chemists to accomplish their total syntheses and that of medicinally relevant anashylogs sometimes in the industrial context [39] Thus tarshygeting the chemical space of natural products has never been more relevant than today Although the discovery of natural products demands time and labor‐consuming manipulations it is worth to notice that the knowledge on this chemical space is still continually growing while biological advances allow for discovering and undershystanding potential targets However increasing the chemical space of human‐made compounds based on natural products should benefit from transdisciplinary colshylaborations such as the use of coupled biosynthetic and chemical synthetic methods to design original ldquounnatural natural productsrdquo [40]
122 The Biosynthetic Pathways as an Inspiration for Synthetic Challenges
1221 Precursor‐Directed Biosyntheses and Mutashysynthetic Strategies to Increase the Chemical Space of Natural Products As the genetics and biochemistry of natural product biosynthesis are better understood novel biosynthetic techniques have been developed to study and generate new diversity in natural product analogs (Scheme 19) Precursor‐directed biosynthesis (PDB) is considered as the earliest example of combining chemical and biological methods for the generation of complex natural product analogs [41 42] This approach compared with the biosynthetic pathway of wild‐type metabolites (Scheme 19a) involves the feeding of analogs of the natural biosynthetic building blocks to the living organisms (Scheme 19b) usually bacteria or fungi which incorporate the modified precursors into the biosynthesized compound mutasynthesis also termed as mutational biosynthesis (mBS) involves the inactivation of a key step of the bioshysynthesis in a mutant microorganism (Scheme 19c) which can then be fed by various modified or advanced building blocks (mutasynthons Scheme 19d) [43] These mutasshyynthons could not be incorporated by the wild type due to specificities of the enzymatic machinery Build up on PDB mBS eliminates the natural biosynthetic intermediate thus generating a less complex mixture of metabolites and making the purification or yield of target products better
O NH
OHO
O
OO
O
O
OH
HOO
O O
OH
HOO
NH ONH2
O
ONH
NH
O
OCl Cl
O
O
OH
HN
HN
HN
HN
OO
HO
HN
OH
OHOH
O
O
HO
HO
OHO
O
OHH2N
O
OH3C
H
O
H
CH3
CH3
H
O O
NO
H
O
HO
O
NO
O
NH
HN
OHO
HONH
ON
H
HO
O
H2N
O OH
ONH
OH
ON OHO
OH
HO OS OH
OOOH
OHO
O
ON
OO
O
HO
O
O OH
OO
Micafunginantifungal
Rapamycinimmunosuppressive
Galantamineanti-Alzheimer
Taxolanticancer
Artemisininantimalarial
Vancomycinantibiotic
FIGURE 15 Some famous natural products currently used as drugs
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product
10 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
different structure strychnine is related to the Corynanthe alkaloids incorporating two carbons from acetyl‐CoA Highly modified monoterpene skeletons are derived from the Corynanthe core through CC bond breaking and reorshyganization leading to Iboga‐type (eg catharanthine) and Aspidosperma‐type (eg vindoline) alkaloids The antishycancer drug vinblastine is a heterodimer resulting from the nucleophilic attack of vindoline on a mannich acceptor resulting from catharanthine found in madagascar perishywinkle (Catharanthus roseus) The heteroaromatic comshypounds ellipticine camptothecin and quinine are also derived from a Corynanthe‐type precursor although in this case the biosynthetic relationship may not be obvious due to deep modifications of the skeleton
Finally two important classes of compounds have to be mentioned since they have inspired many synthetic chemists The pyrroloindole alkaloids result from the cyclization of tryptamine as found in physostigmine (formed by a cationic mechanism after methylation in position 3 of the indole not shown) or in chimonanthine (presumably formed by a radical coupling mechanism Scheme 18) The ergot alkaloids are derived from the 33‐dimethylallylation on position 4 of the indole in trypshytophan whose further cyclization and oxidation processes afford the natural products (eg lysergic acid Scheme 18 and ergotamine) which have had important medical applications [34]
NRPS Metabolites and Peptides NRPS enzymes assemble amino acids including nonproteinogenic ones into oligopeptides The enzymes contain several modules and especially an adenylation domain (A) which specifically selects and activates the amino acid to be transferred as a thioester on the nearby peptidyl carrier protein (PCP) [2] A condensation module (C) then catalyzes the formation of the peptide bonds between the newly introduced amino acyl‐PCP (bearing a free amine) and the elongated peptidyl‐PCP thioester At the end of the elongation a cyclization can occur into cyclopeptides but the peptide can also be
transferred to auxiliary enzymes like methyltransferases glycosyltransferases or oxidases (vancomycins are typical products of such functionalizations) [35 36] The formation of heterocycles is also frequently encountered in this metabolism as in penicillins that are derived from the tripeptide α‐aminoadipoyl‐cysteinyl‐valine or telomestatin (Fig 14) [2]
Other Alkaloid Origins There are many other nitrogen sources involved in alkaloid biosyntheses for example nicotinic acid (originated from aspartic acid and intershymediate in nicotine and anabasine biosyntheses) and anthranilic acid (originated from tryptophan and intermedishyate toward acridines or aurachins) The amination reaction (eg through transamination of carbonyl compounds) is also a way to introduce nitrogens in natural products for example from fatty acids steroids (toward Solanum alkaloids or cyclopamines) or other terpenoids (aconitine and atisine have diterpene skeletons while Daphniphyllum alkaloids are triterpene derivatives) Finally nucleic acids can also be precursors of alkaloids like the well‐known caffeine
12 FROM BIOSYNTHESIS TO TOTAL SYNTHESIS STRATEGIES TOWARD THE NATURAL PRODUCT CHEMICAL SPACE
121 The Chemical Space of Natural Products
Natural products occupy an important place in human com munities as demonstrated by their vast use from ancient times to nowadays like dyes fibers oils pershyfumes agrochemicals or drugs Broadly both primary and secondary metabolites could be classified as natural products while the latter as discussed previously are usushyally regarded as the ldquonatural productsrdquo owing to their comshyplexity and diversity arising from a variety of biosynthetic pathways The structural chemical diversity found among
N
S
CO2HO
HNPhH2C
O
Penicillin G
OH
HN
HN
O
O
ONH2
O
OHO
NHMe
OCl
O
NH
NH
NH
O
ClHO
O
HN
H
H
HOOH
OHHO2C
Vancomycin aglycone
ON
NO
O
NN
O
O
N
N O
Me
S
N N
OMeH
Telomestatin
L-AAA-L-CYS-D-VAL
Enzymes
FIGURE 14 Structural diversity of nonribosomal peptide compounds (AAA α‐aminoadipic acid)
FROm BIOSyNTHESIS TO TOTAL SyNTHESIS STRATEGIES TOWARD THE NATuRAL PRODuCT CHEmICAL SPACE 11
all living organisms defining the chemical space of natural products [37] is the consequence of their evolution occurshyring as an adaptation of organisms to their environment It is commonly believed that secondary metabolites are proshyduced as messengers by living organisms or as weapons against enemies and thus they should have certain biological activities in a medicinal point of view [38] Indeed natural products are regarded as one of the main sources of medicines (Fig 15) From the traditional medicinal extracts to every single bioactive molecule the methods of extraction purification identification and biological investigation of natural products have been well established Their complex structures and interesting properties have attracted synthetic chemists to accomplish their total syntheses and that of medicinally relevant anashylogs sometimes in the industrial context [39] Thus tarshygeting the chemical space of natural products has never been more relevant than today Although the discovery of natural products demands time and labor‐consuming manipulations it is worth to notice that the knowledge on this chemical space is still continually growing while biological advances allow for discovering and undershystanding potential targets However increasing the chemical space of human‐made compounds based on natural products should benefit from transdisciplinary colshylaborations such as the use of coupled biosynthetic and chemical synthetic methods to design original ldquounnatural natural productsrdquo [40]
122 The Biosynthetic Pathways as an Inspiration for Synthetic Challenges
1221 Precursor‐Directed Biosyntheses and Mutashysynthetic Strategies to Increase the Chemical Space of Natural Products As the genetics and biochemistry of natural product biosynthesis are better understood novel biosynthetic techniques have been developed to study and generate new diversity in natural product analogs (Scheme 19) Precursor‐directed biosynthesis (PDB) is considered as the earliest example of combining chemical and biological methods for the generation of complex natural product analogs [41 42] This approach compared with the biosynthetic pathway of wild‐type metabolites (Scheme 19a) involves the feeding of analogs of the natural biosynthetic building blocks to the living organisms (Scheme 19b) usually bacteria or fungi which incorporate the modified precursors into the biosynthesized compound mutasynthesis also termed as mutational biosynthesis (mBS) involves the inactivation of a key step of the bioshysynthesis in a mutant microorganism (Scheme 19c) which can then be fed by various modified or advanced building blocks (mutasynthons Scheme 19d) [43] These mutasshyynthons could not be incorporated by the wild type due to specificities of the enzymatic machinery Build up on PDB mBS eliminates the natural biosynthetic intermediate thus generating a less complex mixture of metabolites and making the purification or yield of target products better
O NH
OHO
O
OO
O
O
OH
HOO
O O
OH
HOO
NH ONH2
O
ONH
NH
O
OCl Cl
O
O
OH
HN
HN
HN
HN
OO
HO
HN
OH
OHOH
O
O
HO
HO
OHO
O
OHH2N
O
OH3C
H
O
H
CH3
CH3
H
O O
NO
H
O
HO
O
NO
O
NH
HN
OHO
HONH
ON
H
HO
O
H2N
O OH
ONH
OH
ON OHO
OH
HO OS OH
OOOH
OHO
O
ON
OO
O
HO
O
O OH
OO
Micafunginantifungal
Rapamycinimmunosuppressive
Galantamineanti-Alzheimer
Taxolanticancer
Artemisininantimalarial
Vancomycinantibiotic
FIGURE 15 Some famous natural products currently used as drugs
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product
FROm BIOSyNTHESIS TO TOTAL SyNTHESIS STRATEGIES TOWARD THE NATuRAL PRODuCT CHEmICAL SPACE 11
all living organisms defining the chemical space of natural products [37] is the consequence of their evolution occurshyring as an adaptation of organisms to their environment It is commonly believed that secondary metabolites are proshyduced as messengers by living organisms or as weapons against enemies and thus they should have certain biological activities in a medicinal point of view [38] Indeed natural products are regarded as one of the main sources of medicines (Fig 15) From the traditional medicinal extracts to every single bioactive molecule the methods of extraction purification identification and biological investigation of natural products have been well established Their complex structures and interesting properties have attracted synthetic chemists to accomplish their total syntheses and that of medicinally relevant anashylogs sometimes in the industrial context [39] Thus tarshygeting the chemical space of natural products has never been more relevant than today Although the discovery of natural products demands time and labor‐consuming manipulations it is worth to notice that the knowledge on this chemical space is still continually growing while biological advances allow for discovering and undershystanding potential targets However increasing the chemical space of human‐made compounds based on natural products should benefit from transdisciplinary colshylaborations such as the use of coupled biosynthetic and chemical synthetic methods to design original ldquounnatural natural productsrdquo [40]
122 The Biosynthetic Pathways as an Inspiration for Synthetic Challenges
1221 Precursor‐Directed Biosyntheses and Mutashysynthetic Strategies to Increase the Chemical Space of Natural Products As the genetics and biochemistry of natural product biosynthesis are better understood novel biosynthetic techniques have been developed to study and generate new diversity in natural product analogs (Scheme 19) Precursor‐directed biosynthesis (PDB) is considered as the earliest example of combining chemical and biological methods for the generation of complex natural product analogs [41 42] This approach compared with the biosynthetic pathway of wild‐type metabolites (Scheme 19a) involves the feeding of analogs of the natural biosynthetic building blocks to the living organisms (Scheme 19b) usually bacteria or fungi which incorporate the modified precursors into the biosynthesized compound mutasynthesis also termed as mutational biosynthesis (mBS) involves the inactivation of a key step of the bioshysynthesis in a mutant microorganism (Scheme 19c) which can then be fed by various modified or advanced building blocks (mutasynthons Scheme 19d) [43] These mutasshyynthons could not be incorporated by the wild type due to specificities of the enzymatic machinery Build up on PDB mBS eliminates the natural biosynthetic intermediate thus generating a less complex mixture of metabolites and making the purification or yield of target products better
O NH
OHO
O
OO
O
O
OH
HOO
O O
OH
HOO
NH ONH2
O
ONH
NH
O
OCl Cl
O
O
OH
HN
HN
HN
HN
OO
HO
HN
OH
OHOH
O
O
HO
HO
OHO
O
OHH2N
O
OH3C
H
O
H
CH3
CH3
H
O O
NO
H
O
HO
O
NO
O
NH
HN
OHO
HONH
ON
H
HO
O
H2N
O OH
ONH
OH
ON OHO
OH
HO OS OH
OOOH
OHO
O
ON
OO
O
HO
O
O OH
OO
Micafunginantifungal
Rapamycinimmunosuppressive
Galantamineanti-Alzheimer
Taxolanticancer
Artemisininantimalarial
Vancomycinantibiotic
FIGURE 15 Some famous natural products currently used as drugs
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product
12 FROm BIOSyNTHESES TO TOTAL SyNTHESES AN INTRODuCTION
Both approaches can potentially greatly increase the divershysity of natural compounds
1222 The Biomimetic Strategy A Bridge between Biosynthesis and Total Synthesis During the past century synthetic chemists were endeavoring to discover more effishycient strategies to access complex natural products The chemical synthesis of tropinone by Robinson in 1917 [44] one of the first biomimetic ones is a fantastic example of an early efficient synthesis which consisted in a multicomposhynent process between succinaldehyde methylamine and calcium acetonedicarboxylate [45] Since then the conshystruction of natural products by chemical methods inspired by naturersquos biosynthetic pathways has attracted many synthetic chemists and participated in the progress of organic chemistry As discussed in the book Biomimetic Organic Synthesis coedited by one of us (BN) an increasing number of total syntheses have been termed ldquobiomimeticrdquo or ldquobioinshyspiredrdquo during the last 20 years meaning the use of a synthetic tactic that follows or mimics a hypothetical or proven biosynthetic pathway Concomitantly the biosynshythesis of natural products has been more and more undershystood thanks to genetic and enzymatic studies Therefore as a bridge between biosynthesis and total synthesis biomishymetic synthesis is able to overcome some drawbacks of conshyventional strategies as it often relies on the self‐assembling properties of a key reactive intermediate [46]
Tremendous works dealing with bioinspired total synshytheses of secondary metabolites have thus been achieved providing new insights in the reactivity of biomimetic precursors and occasionally leading to controversy or
unresolved questions [47] An interesting example goes to hirsutellones a family of fungal PKSNRPS compounds (also regarded as alkaloids due to their nitrogen) with intriguing structures and a significant antitubercular activity [7] Their biosynthesis has been hypothesized by Oikawa who proposed a key linear precursor of the related compounds GKK1032A
2 made from one tyrosine nine
AcCoA and several methylations by S‐adenosylmethionine [48] We applied this hypothesis to the less methylated hirsutellones (Scheme 110) Two different biosyn thetic pathways can be proposed for the polycyclization Pathway (a) involves the selective oxidation of one of the dienoyl double bonds (CγCδ) to generate an epoxide and of the phenol This ldquoelectrophilic headrdquo would then be attacked in a conjugated ene reaction involving the triene and initishyating the cyclization Formation of the bent paracycloshyphane would then be followed by a stereoselective intramolecular DielsndashAlder (ImDA) reaction leading to the complete tricyclic core of the natural product Pathway (b) involves the allylic oxidation at the terminal methyl group of the triene to release an allylic alcohol or cation as an ldquoelectrophilic tailrdquo The polycyclization would then be initiated through reverse electronic activation compared to pathway (a) forming the first cyclohexane ring before the ImDA reaction occurs
Nicolaou et al [49] and uchiro et al [50] achieved the total syntheses of hirsutellone B in 2009 and 2011 respecshytively We recently described a formal total synthesis by forming the key decahydrofluorene (tricyclic) core of hirsutellone in a biomimetic strategy following pathway (b) [47] As for the synthesis of this important synthetic
E1
(a)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
E1 E2 E3A
A
B
B CC
B
A AʹBʹCʹ
(b)
Usually as a mixture with the wild-type intermediates depending on
enzyme specicity
E1
(c)
E2 E3A
A
B
B CC
B
A AʹBʹCʹ
The AndashB intermediate is notproduced the biosynthesis is blocked
Blocked mutant
E1
(d)
E2 E3A
A
B
C
The mutasynthon B replaces themissing natural intermediate B
and is incorporated in the productBlocked mutant
E4D
BE4
D
E4D B
BD
C
B
A AʹBʹCʹ
A
B+
C
B
A+
AʹBʹCʹ
+
E4
SCHEME 19 (a) Biosynthetic pathway of wild‐type metabolites (b) precursor‐directed biosynthesis the modified synthon B replaces the natural synthon B (c) biosynthetic pathway blocked by a mutation (the enzyme E4 is not functional) (d) mutasynthesis a mutasynthon B is introduced to replace B and is incorporated in the biosynthesis leading to a ldquomutatedrdquo natural product